HYDROMEDUSAE , a See also:

• GROUP

The species of Hydra, however, are extremely common and See also:

• FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family)

Groups founded on polyp-characters are printed in See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

Hydrocorallinae. „ IV. Graptolitoidea. V. Trachylinae. Sub-order r. Traehomedusae. 2. Narcomedusae. Order VI. Siphonophora. Sub-order I.

Chondrophorida. 2. Calycophorida. 3. Physophorida. 4. Cystophorida. Organization and See also:

• MORPHOLOGY, (Gr. yopdsil, form)

The Hydropolyp (fig. I)—The general characters of this organism are described above and in the articles HYDROZOA and See also:

• PONY
• PONY (from the Lowland Scots powney, probably from 0. Fr. pouleriet, diminutive of poulain, a colt or foal; Late Lat. pullanus, Lat. pullus, a young animal)

J. See also:

• ALLMAN, GEORGE JAMES (1812-1898)

Amphibrachium grows in the tissues of a sponge, Euplectella, and protrudes its hydranth into the See also:

• CANAL
• CANAL (from Lat. canalis, " channel " and " kennel " being doublets of the word)

idsum (fig. 2) several; in Coryne and Cordylophora the tentacles are scattered irregularly over the elongated hydranth. As regards form, the tentacles show a number of types, of which the most important are (I) filiform, i.e. cylindrical or tapering from 'The See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

In the curious polyp Myriothela the body of the polyp is differentiated into nutritive and reproductive portions. See also:

• HISTOLOGY
• HISTOLOGY (Gr. Ler6c, web, tissue, properly the web-beam of the loom, from iQravai, to make to stand)

The sub-epithelial layer consists primarily of the so-called interstitial cells, lodged between the narrowed basal portions of the epithelial cells. From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or See also:

• MOTHER

The outer wall of the capsule is in-complete at one See also:

• POLE (FAMILY)
• POLE, REGINALD (1500-1558)
• POLE, RICHARD DE LA (d. 1525)
• POLE, WILLIAM (1814—1900)

A third point of dispute is whether the nematocysts are formed in situ, or whether the cnidoblattts migrate with them to the region where they' are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view. that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells. The muscular See also:

• TISSUE (Fr. tissu, tissue, participle of tisser, Lat. texere, to weave)

C. Chun, HYDROZOA PP. 314, 315)- The mesogioea in the hydropolyp is a thin elastic layer, in whichmaybe lodged the muscular fibres and ganglion cells mentioned above. but which never contains any connective tissue or skeletogenous cells or any other See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

In other cases, however, the medusa-individuals become sexually mature while still attached to the parent polyp, and are then not set free at all, but become appanages of the hydroid colony and undergo degenerative changes leading to reduction and even to complete obliteration of their See also:

• ORIGINAL

Each bud produced function of circulating and distributing nutriment through the colony. The hydroid colony shows many variations in form and architecture which depend simply upon differences in the methods in which polyps are budded. In the first See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

A and B modified cation is seen when the from Hincks; C modified from Forbes's Brit. Naked-eyed Med tubes of the basal urae. perisarc do not remain of its Medusa, Willia stellata. A, colony of but grow in all planes See also:

• LAS

In the formation of arborescent colonies, two sharply distinct types of budding are found, which are best deseribed in botanical terminology as the monopodial or racemose, and the sympodial or cymose types respectivell, ; each is characteristic of one of the two sub-orders of the Hydrrndea, the Gynlnoblastea and Calyptoblastea. In the monopodial method (figs.' i2, 14) the founder-polyp is, by the founder proceeds to grow and to bud in the. same way as the founder did, producing a See also:

• SIDE
• SIDE (mod. Eski Adalia)

17, biserial sbudding; eachnpolyp budding, uniserial type, shown on the main stem forms a in four stages (1-4). F, founder- second' bud, which usually polyp; I, 2, 3, See also:

• SUCCESSION (Lat. successio, from succedere, to follow after)

5), the blastostyles gradually lose their nutritive function and the organs connected with it; the mouth and tentacles disappear, and the blastostyle obtains the nutriment necessary for its activity by way of the coenosarc. In the Calyptoblastea, where the polyps are protected by special capsules of the perisarc, the gonothecae en-closing the blastostyles differ from the hydro-thecae protecting the hydranths (fig. 54). In other colonies the two functions of the nutritive polyp, namely, See also:

• CAPTURE (from Lat. capere, to take; Fr. prise maritime; Ger. Wegnahme)

Besides the three types of individual above mentioned, there are other appendages of hydroid colonies, of which the individuality is doubtful. Such are the " guard-polyps " (machopolyps) of Plumularidae, which are often regarded as individuals of the nature of dactylozoids, but from a study of the mode of budding in this hydroid See also:

• FAMILY

As regards See also:

• HABIT (through the French from Lat. habitus, from habere, to have, hold, or, in a reflective sense, to be in a certain condition; in many of the English senses the French use habitude, not habit)

No other instances are known of sessile habit in Hydromedusae. The external form of the Hydromedusae varies from that of a deep See also:

• BELL
• BELL, ALEXANDER MELVILLE (1819—1905)
• BELL, ANDREW (1753—1832)
• BELL, GEORGE JOSEPH (1770-1843)
• BELL, HENRY (1767-1830)
• BELL, HENRY GLASSFORD (1803-1874)
• BELL, JACOB (1810-1859)
• BELL, JOHN (1691-178o)
• BELL, JOHN (1763-1820)
• BELL, JOHN (1797-1869)
• BELL, ROBERT (1800-1867)
• BELL, SIR CHARLES (1774—1842)

23 and 25) ; and, fourthly, in structure, being hollow or solid, as in the polyp. In some medusae, for instance, the remarkable deep-sea family Pectyllidae, the tentacles may bear suckers, by which the animal may attach itself temporarily. It should be mentioned t finally that the tentacles are very contractile and extensible, and may therefore present themselves, in one and the same Individual, as long, drawn-out threads, or in the form of short corkscrew - like ringlets; they may stream downwards from the sub-umbrella, or be held out horizontally, or be directed upwards over the ex-umbrella (fig. 23). Each species of After O, See also:

• MAAS, JOSEPH (1847-1886)

The internal anatomy of the Hydromedusae shows numerous variations. The stomach may be altogether lodged in the manubrium, from which the radial canals then take origin directly as in Geryonia (Trachomedusae) ; it may be with or without gastric pouches. The radial canals may be simple or branched, primarily four, rarely six in number. The See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

See also:

• SOC

Peronia or cartilaginous process ascending from the cartilaginous margin of the disk centripetally in the outer surface of the jelly-like disk; six of these are perradial, six interradial, corresponding to the twelve solid larval tentacles, resembling those of Cunina. k, Dilatation (stomach) of the manubrium. 1, Jelly of the disk. p, Manubrium. t, Tentacle (hollow and See also:

• TERTIARY

In contrast with the polyp, the longitudinal muscle-system is entirely ectodermal, there being no endodermal muscles in s..aspedote medusae. The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerve-rings at the margin of the umbrella, near the base of the velum. One, the " upper " forms part of the epidermal See also:

• MOSAIC (corresponding to Lat. opus musivum, from Gr. µovoeiov, an artificial grotto often decorated with mosaics; the word is only found in the sense of mosaic in late Greek, which generally uses k oXI yrlµa)

30). The sense-organs may be tentaculocysts, i.e. modifications of a tentacle, as in Trachylinae, or developed from the margin of the umbrella, in no connexion with a tentacle (or, if so connected, not producing any modification in the tentacle), as in Leptolinae. In Hydromedusae the sense-organs are always exposed at the umbrellar margin (hence Gymnophthalmata), while rn Scyphomedusae they are covered over by flaps of the umbrellar margin (hence Steganophthalmata). The statocysts present in general the structure of either a knob or a closed vesicle, composed of (i) indifferent supporting epithelium; (2) sensory, so-called auditory epithelium of slender cells, each of the medusa differs only in greater elaboration and differentiation of the cell-elements, which are also more concentrated to form distinct tissues. The ectoderm furnishes the general epithelial covering of the body, and the muscular tissue, nervous system and sense-organs. The is-S- Fm. 26.-Carmarina (Geryonia) hastata, one of the Trachomedusae. (After See also:

• HAECKEL, ERNST HEINRICH (1834- )

The nematocysts of the ectoderm may be grouped to form batteries on the tentacles, umbrellar margin and oral lappets. In places the nematocysts may be crowded so thickly as to form a tough, supporting, "chondral " tissue, resembling See also:

• CARTILAGE (Lat. cartilage, gristle)

The. pit has its opening turned towards the sub-umbral cavity, while its Z~ -• " st.c.~'~ base or fundus forms abulge, 'con more or less pronounced, on Modified after O. and R. Hertwig, Nerves- the ex-umbral side. of the system and Sinnesorgane der Medusas, by velum. At the fundus are permission of F. C. W. See also:

• VOGEL, EDUARD (1829-1856)
• VOGEL, SIR JULIUS (1835-1899)

st.c, Cavity of statocyst. vesicle (the fundus of the original pit) or on its sides; their arrangement and number vary greatly and furnish useful characters for distinguishing genera. The sense-cells are innervated, as before, from the sub-umbral nerve-ring. The inner wall of the vesicle (region of See also:

• CLOSURE (Fr. clooture)

36), where the ectoderm of the cushion rises up in a double See also:

• FOLD

ordinary indifferent cells of the epithelium containing pigment-granules, and (2) visual cells, slender sensory epithelial cells of the usual type, which may develop visual cones or rods at their free extremity. The ocelli occur usually either on the inner or outer sides of the tentacles; if on the inner side, the tentacle is turned upwards and carried over the ex-umbrella, so as to expose the ocellus to the light; if the FIG. 36.-Simple tentaculocyst of Rhopalo• ocellus be on the sterna velatum. The process carrying the otolith outer side of a or See also:

• CONCRETION

Modified after O. and R. Hertwig, Nerves-system and Sinnesorgane der Medusen, by permission of F. C. W. Vogel. ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. v, Velum. six, Cavity of statocyst. con, Concrement-cell with otolith. V con, Modified after O. and R. Hertwig, A'ervensystem and Sinnesorgane der Medusen, by permission of F.

C. W. Vogel. a Statocyst of Oclorchis. with After O. and R. Hertwig, Nervensystem and Sinnesorgane der Medusen, by permission of F. C. W. Vogel. body. The distal portion of the vitreous body may project from the cavity of the cup, forming a non-cellular See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

Beyond this simple condition the visual organs of the Hydromedusae do not advance, and are far from reaching the wonderful development of the eyes of Scyphomedusae (Charybdaea). Besides the ordinary type of ocellus just described, there is found in one genus(Tiaropsis) a type of ocellus in which the visual elements are inverted, and have their cones turned away from the light, as in the human retina (fig. 30). In this case the pigment-cells are endodermal, forming a cup of pigment in which the visual cones are embedded. A similar ocellus is formed in See also:

• AURELIA, VIA

nrq, Sub-umbral nerve-ring. ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. c.c, Circular canal. spicuous or absent v, Velum. (fig. 39). Each cordylus is a ten- tacle-like structure with an endodermal axis containing. an axial cavity which may be continuous with the ring-canal, or may be partially occluded. Externally the cordylus is covered by very flattened ectoderm, and bears no otoliths or sense-cells, but the base of the club rests upon the ex-umbral nerve-ring. See also:

• BROOKS, CHARLES WILLIAM SHIRLEY (1816-1874)
• BROOKS, PHILLIPS (1835-1893)

The endoderm of the medusa shows the same general types of structure as in the polyp, described above. We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu- latory endoderm, in the radial and ring- canals; (3) supporting endoderm in the axes of the tentacles and in the endoderm- lamella; the latter is primitively a double layer of cells, produced by concrescence of the ex-umbral and sub-umbra] layers of the coelenteron, but it is usually found as a single layer of flattened cells (fig. 40) ; in Geryonia, however, it remains double, and the centripetal canals arise by parting of the two layers; (4) excretory endoderm, lining pores at the margin of the umbrella, occurring in certain Leptomedusae as so- called " marginal tubercles," opening, on the one hand, into the ring-canal and, on the other hand, to the exterior by " marginal funnels," which debouch into the sub-umbral cavity above the velum. As has been de- scribed above, the endoderm may also con- tribute to the sense-organs, but such contributions are always of an See also:

• ACCESSORY

The non-sexual reproduction takes the form of fission, budding or sporogony, the details of which are described below. Buds may be produced from the manubrium, radial canals, ring-canal, or tentacle-bases, or from an aboral stolen (Narcomedusae). In all cases only medusa-buds are produced, never polyp-buds. The mesogloea of the medusa is largely developed and of great thickness in the umbrella. The sub-epithelial tissues, i.e. the nervous and muscular cells, are lodged in the mesogloea, but in Hydromedusae it never contains tissue-cells or mesogloeal corpuscles. (b) The Medusae as a Subordinate Individuality. —It has been shown above that polyps are budded only from polyps and that the medusae may be budded either from polyps or from medusae. in any case the daughter-individuals produced from the buds may be imagined ae remaining attached to the parent and forming a colony of individuals in organic connexion with one another, and thus three possible cases arise. The first case gives a colony entirely composed of polyps, as in many Hydroidea: The second case gives a colony partly composed of polyp-individuals, partly of medusa-individuals, a possibility also realized in many colonies of Hydroidea. The third case gives a colony entirely composed of medusa-individuals, a possibility perhaps realized in the Siphonophora, which will be discussed in dealing with this group. The first step towards the formation of a mixed hydroid colony is undoubtedly a hastening of the sexual maturity of the medusa-individual. Normally the medusae are liberated in quite an immature See also:

• STATE
• STATE, GREAT OFFICERS OF

It may be, however, that in-creased reproductiveness becomes of greater importance to the species than wide diffu- See also:

• SION [Ger. Sitten]

Brooks, See also:

• JOURNAL
• JOURNAL (through Fr. from late Lat. diurnalis, daily)

A very similar condition is seen in Tubularia (fig. 41, B), where, however, the tentacles have quite disappeared, and the circular rim formed by the margin of the umbrella has nearly closed over the manubrium leaving only a small aperture through which the embryos emerge. The next step is illustrated by the female gonophores of Cladocoryne, where the radial and ring-canals Modified from Weisman, Entslehung der Sexualzelten bei den Hydromedusen. A, " Meconidium" of Gonothyraea. H, Withspadixbranched(Cordy- B, Type of Tubularia. lophora). C, Type of Garveia, &c. [&c. s.c, Sub-umbral cavity. D, Type of Plumularia, Agalma, t, Tentacles. E, Type of Coryne, Forskalia,&c. c.c, Circular canal. F, G, H, Sporosacs. g, Gonads. F, With simple spadix. sp, Spadix. G, With spadix prolonged c.l, Endoderm-lamella.

(Eudendrium). ex, Ex-umbral ectoderm. ect, Ectotheca. have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endoderm-lamella. Next the opening of the umbrella closes up completely and disappears, so that the sub-umbral cavity forms a closed space surrounding the manubrium, on which the gonads are developed; such a condition is seen in the male gonophore of Cladocoryne and in Garveia (fig. 41, C), where, however, there is a further complication in the form of an See also:

• ADVENTITIOUS (from Lat. adventicius, coming from abroad)

The gonophore is now reduced to its simplest condition, known as the sporosac (fig. 41, F, G, H), and consists of the spadix bearing the gonads covered by a single layer of ectoderm (ex.), with or without the addition of an ectotheca. It cannot be too strongly emphasized, however. that the sporosac should not be compared simply with the manubrium of the medusa, as is sometimes done. The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared. The spadix, as the organ of nutrition for the gonads, may be developed in various ways, being simple (fig. 1, F) or branched (fig. 41, H); in Eudendrium (fig. 4r, G) it curls round the single Large ovum. The hydroid Dicoryne is remarkable for the possession of gonophores, which are ciliate and become detached and swim away by means of their cilia. Each such sporosac has two long tentacle-like processes thickly ciliated. It has been maintained that the gonads of Hydra represent sporosacs or gonophores greatly reduced, with the last traces of medusoid structure completely obliterated. There is, however, no See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

Hydra is, moreover, bisexual, In contrast with what is known of hydroid colonies. In some Leptomedusae the gonads are formed on the radial canals and form protruding masses resembling sporosacs superficially, but not in structure. Allman, however, regarded this type of gonad as See also:

• EQUIVALENT

ov, Ova, two carried on each female gonophore. sp, Testis. (histocytes) and germinal cells, actual or potential (archaeocytes), amongst the constittient cells of the animal body. In this way we may distinguish, first, vegetative reproduction, the result of discontinuous growth of the tissues and cell-layers of the body as a whole, leading to (r) fission, (2) autotomy, or (3) vegetative budding; secondly, germinal reproduction, the, result of the reproductive activity of the archaeocytes or germinal tissue. In germinal reproduction the proliferating cells may be undifferentiated, so-called primitive germ-cells, or they may be differentiated as sexual cells, male or female, i.e. spermatozoa and ova. If the germ-cells are undifferentiated, the offspring may arise from many cells or from a single cell; the first type is (4) germinal budding, the second is (5) sporogony. If the germ-cells are differentiated, the offspring arises by syngamy or sexual union of the ordinary type between an ovum and spermatozoon, so-called. fertilization of the ovum, or. by parthenogenesis, i.e. development of an ovum without fertilization. The only one of these possible modes of reproduction not known to occur in Hydromedusae is parthenogenesis. (i) True fission or longitudinal See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

By budding is understood the formation of a new individual from a fresh growth of undifferentiated material. It is convenient to distinguish buds that give rise to polyps from those that form medusae. (a). The Polyp.—The buds that form polyps are very simple in mode of formation. Four stages may be distinguished; the first is a simple outgrowth of both layers, ectos.c. See also:

• DERMA

Radial canal. tion of a stage c.c, Circular canal. similar to D. e.l, Endoderm lamella. Gc, Entocodon. m, Manubrium. s.c, Cavity of ento- v, Velum. codon, forming t, Tentacle. the future bulge out laterally near the See also:

• ATTACHMENT

45, A). The ectodermal ingrowth is the entocodon (Gs.) ; it bulges into, and pushes down, the endoderm at the apex of the bud, and if solid it soon acquires a cavity (fig. 44, C, s.c.). The cavity of the entocodon increases continually in size, while: the endoderm pushes up at the sides of it to form a cup with hollow walls, enclosing but not quite surrounding the entocodon, which remains in contact at its outer side with the ectoderm covering the bud (fig. 44, D, v). The next changes that take place are chiefly In the endoderm-cup (fig. 44, D, E) ; the cavity A 8 C between the two walls of the cup FIG. 45.—Modifications of the method of becomes reduced budding shown in fig. 44, with solid Entoby concrescence to codon (Gc.) and formation of an ectotheca (See also:

• EA(nar...), (p.s„,.,.)
• EA(psr...), (p, „...)• (pqr...)