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England. morphological peculiarities underlying the utmost diversity of Thus a planula larva may be a blastula, or but slightly advanced beyond this See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage, or it may be (and most usually is) a parenchymula; or in some cases (See also:

SCYPHOMEDUSAE

Scyphomedusae) it may be a gastrula. It should be added that the See also:

PROCESS

process of development, the gastrulation as it is termed, may be shortened by the See also:

IMMIGRATION (from Lat. in, into, and tnigrare, to depart)

immigration of cells taking See also:

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

place superficial characters. All Hydrozoa, in the first place, exhibit the three structural features distinctive of the See also:

COELENTERA

Coelentera (q.v.). (I) The See also:

BODY

body is built up of two layers only, an external protective and sensory layer, the ectoderm, and an See also:

INTERNAL

internal See also:

DIGESTIVE

digestive layer, the endoderm. (2) The body contains but a single internal cavity, the coelenteron or gastrovascular space, which may be greatly ramified, but is not shut off into cavities distinct from the central digestive space. (3) The generative cells are produced in either the ectoderm or endoderm, and not in a third layer arising in the embryo, distinct from the two See also:

PRIMARY

primary layers; in other words, there is no mesoderm or coelom. To these three characters the Hydrozoa add a See also:

FOURTH

fourth which is distinctive of the subdivision of the Coelenterata termed the Cnidaria; that is to say, they always possess See also:

PECULIAR

peculiar stinging See also:

ORGANS

organs known as See also:

NETTLE (0. Eng. netele, cf. Ger. Nessel)

nettle-cells, or nematocysts (Cnidae), each produced in a See also:

CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell forming an integral part of the animal's tissues.

The Hydrozoa are thus shown to belong to the group of Coelenterata Cnidaria, and it remains to consider more fully their distinctive features, and in particular those which See also:

mark them off from the other See also:

main See also:

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

division of the Cnidaria, the See also:

ANTHOZOA (i.e. " flower-animals ")

Anthozoa (q.v.), comprising the corals and See also:

sea-anemones. The See also:

GREAT

great diversity, to which reference has already been made, in the form and structure of the Hydrozoa is due to two See also:

PRINCIPAL

principal causes. In the first place, we find in this group two distinct types of See also:

PERSON, OFFENCES AGAINST THE

person or individual, the See also:

POLYP

polyp and the See also:

MEDUSA

medusa (qq.v.), each capable of a wide range of See also:

variations; and when both polyp and medusa occur in the See also:

LIFE

life-See also:

cycle of the same species, as is frequently the See also:

case, the result is an See also:

ALTERNATION (from Lat. alternare, to do by turns)

alternation of generations of a type peculiarly characteristic of the class. In the second place, the See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power of non-sexual See also:

REPRODUCTION

reproduction by budding is practically of universal occurrence among the Hydrozoa, and by the buds failing to See also:

SEPARATE

separate from the See also:

PARENT, SIMON NAPOLEON (1855– )

parent stock, colonies are produced, more or less complicated in structure and often of great See also:

SIZE

size. We find that polyps may either bud other polyps or may produce medusae, and that medusae may bud medusae, though never, apparently, polyps. Hence we have a primary subdivision of the colonies of Hydrozoa into those produced by budding of polyps and those produced by budding of medusae. The former may contain polyp-persons and medusa-persons, either one See also:

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

kind alone or both kinds combined; the latter will contain only medusa-persons variously modified. The See also:

MORPHOLOGY, (Gr. yopdsil, form)

morphology of the Hydrozoa reduces itself, therefore, to a See also:

CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

consideration of the morphology of the polyp, of the medusa and of the See also:

COLONY (Lat. colonia, from colonus, a cultivator)

colony. Putting aside the last-named, for a detailed account of which see HYDROMEDUSAB, we can best See also:

DEAL

deal with the peculiarities of the polyp and medusa from a developmental point of view. In the development of the Hydrozoa, and indeed of the Cnidaria generally, the See also:

egg usually gives rise to an See also:

OVAL (Lat. ovum, egg)

oval larva which swims about by means of a coating of See also:

CILIA (plural of Lat. cilium, eyelash)

cilia on the See also:

SURFACE

surface of the body. This very characteristic larva is termed a planula, but though very uniform externally, the planulae of different species, or of the same species at different periods, do not always represent the same stage of embryonic development internally. On examining more minutely the course of the development, it is found that the ovum goes through the usual process of cleavage, always See also:

TOTAL

total and See also:

REGULAR

regular in this group, and so gives rise to a hollow See also:

SPHERE (Gr. vclsa?pa, a ball or globe)

sphere or ovoid with the See also:

wall composed of a single layer of cells, and containing a spacious cavity, the blastocoele or segmentation-cavity.

This is the blastula stage occurring universally in all Metazoa, probably representing an ancestral Protozoan colony in phylogeny. Next the blastula gives rise to an internal See also:

mass of cells (fig. I, hy) which come from the wall either by immigration (fig. 1, A) or by splitting off (delamination). The formation of an inner cell-mass converts the single-layered blastula (monoblastula) into a See also:

double-layered embryo (diblastula) which may be termed a parenchymula, since at first the inner cell-mass forms an irregular parenchyma which may entirely fill up and obliterate the segmentation cavity (fig. 1, B). At a later stage, however, the cells of the inner mass arrange them-selves in a definite layer surrounding an internal cavity (fig. 1, C, al), which soon acquires an opening to the exterior at one See also:

pole, and so forms the characteristic embryonic stage of all Enterozoa known as the gastrula (fig. 2). In this stage the body is composed of two layers, ectoderm (d) externally, and endoderm (c) internally, surrounding a central cavity, the archenteron (b), which communicates with the exterior by a See also:

PORE

pore (a), the blastopore, C From See also:

Balfour, after Kowalewsky. at one pole only, and in a connected layer with orderly arrangement, so that the gastrula stage is reached at once from the blastula without any intervening parenchymula stage. This is a process of gastrulation by invagination which is found in all animals above the Coelenterata, but which is very rare in the Cnidaria, and is known only in the Scyphomedusae amongst the Hydrozoa.

After the gastrula stage, which is found as a developmental stage in all Enterozoa, the embryo of the Hydrozoa proceeds to develop characters which are peculiar to the Coelen- a terata only. See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

Round the blastopore hollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm. Each outgrowth contains a prolongation of the archenteric cavity (compare See also:

FIGS

figs. 2 and 3, A). In this way is formed a 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

ring of tentacles, the most characteristic organs of the Cnidaria. They surround a region which is termed the peristome, and which contains in the centre the blastopore, which becomes the adult mouth. The archenteron becomes the gastrovascular See also:

SYSTEM

system or coelenteron. Between the ectoderm and endoderm a gelatinous supponing layer, Feats of rom GegenCbaurp om'sarative EIem termed the mesogloea, makes its appearance. ,tnalomy. The gastrula has now become an actinula, Fin. 2.—See also:

DIAGRAM

Diagram which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a a, Blaofsa topore. Diblastula. transitory manner the common ancestor of b, Blasco riccavity. the group.

In no case known, however, does c, Endoderm. the actinula become the adult, sexually mature d, Ectoderm. individual, but always undergoes further modifications, whereby it develops into either a polyp or a medusa. To become a polyp, the actinula (fig. 3, A) becomes attached to some See also:

FIRM

firm See also:

OBJECT

object by the pole farthest from the mouth, and its growth preponderates in the direction of the principal See also:

AXIS (Lat. for " axle ")

axis, that is to say, the axis passing through the mouth (fig. 3, a-b). As a result the body becomes columnar in form (fig. 3, B), and without further See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change passes into the characteristic polyp-form (see Polyp). a a. It is convenient to distinguish two types of polyp by the names hydro polyp and anthopolvp, characteristic of the Hydrozoa and Anthozoa respectively. In the hydropolyp the body is typically elongated, the height of the See also:

COLUMN (Lat. columna)

column being far greater than the See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter. The peristome is relatively small and the mouth is generally raised on a projecting spout or hypostome. The ectoderm loses entirely the ciliation which it had in the planula and actinula stages and commonly secretes on its external surface a protective or supporting in-vestment, the perisarc. Contrasting with this, the anthopolyp is generally of squat form, the diameter often exceeding the height; the peristome is wide, a hypostome is lacking, and the ectoderm, or so much of it as is exposed, i.e. not covered by secretion of skeletal or other in/estment, retains its ciliation throughout life.

The internal structural differences are even more characteristic. In the hydropolyp the blastopore of the embryo forms the adult mouth situated at the extremity of the hypostome, and the ectoderm and a. R R Fie. 4.-Diagram showing the change of the Actinula into a Medusa. A, See also:

VERTICAL (from Lat. vertex, highest point)

Vertical See also:

section of the actinula; a-b and c-d as in fig. 3, B, transitional stage, showing preponderating growth in the See also:

HORIZONTAL

horizontal See also:

PLANE

plane. C',C', D,D', two types of medusa organization; C and D are composite sections, showing a See also:

RADIUS

radius (R) on one See also:

side, an interradius (I R) on the other; C' and D' are plans; the mouth and manubrium are indicated at the centre, leading into the gastral cavity subdivided by the four areas of concrescence in each interradius (IR). t, tentacle; g.p, gastric pouch; r.c, radial See also:

canal not See also:

PRESENT

present in C and C'; c.c, circular or ring-canal; e.l, endoderm-lamella formed by concrescence. For a more detailed diagram of medusa-structure see See also:

ARTICLE (from Lat. articulus, a joint)

article MEDUSA. endoderm meet at this point. In the anthopolyp the blastopore is carried inwards by an in-pushing of the body-wall of the region of the peristome, so that the adult mouth is an opening leading into a See also:

SHORT, FRANCIS JOB (1857– )

short ectodermal See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

oesophagus or stomodacum, at the bottom of which is the blastopore. Further, in the hydropolyp the digestive cavity either remains See also:

SIMPLE

simple and undivided and circular in transverse section, or may show ridges projecting internally, which in this case are formed of endoderm alone, without any participation of the mesogloea. In the anthopolyp, on the other See also:

hand, the digestive cavity is always subdivided by so-called mesenteries, in-growths of the endoderm containing vertical lamellae of mesogloea (see ANTUOZOA).

In short, the hydropolyp is characterized by a more simple type of organization than the anthopolyp, and is in most respects less modified from the actinula type of structure. Returning now to the actinula, this form may, as already stated, develop into a medusa, a type of individual found only in the Hydrozoa, as here understood. To become a medusa, the actinula grows scarcely at all in the direction of the principal axis, but greatly along a plane at right angles to it. Thus the body becomes See also:

UMBRELLA

umbrella-shaped, the See also:

CONCAVE

concave side representing the peristome, and the See also:

CONVEX

convex side the column, of the polyp. Hence the tentacles are found at the edge of the umbrella, and the hypostome forms usually a projecting See also:

TUBE (Lat. tuba)

tube, with the mouth at the extremity, forming the manubrium or handle of the umbrella. The medusa has a pronounced radial symmetry, and the positions of the primary tentacles, usually four in number, mark out the so-called radii, alternating with which are four interradii. The ectoderm retains its ciliation only in the sensory organs. The mesogloea becomes enormously increased in quantity (hence the popular name " jelly-See also:

fish "), and in correlation with this the endoderm-layer lining the coelenteron becomes pressed together in the interradial areas and undergoes concrescence, forming a more or less complicated gastrovascular system (see MEDUSA). It is sufficient to See also:

state here that the medusa is usually a See also:

FREE

free-See also:

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming animal, floating mouth downwards on the open seas, but in some cases it may be attached by its aboral pole, like a polyp, to some firm basis, either temporarily or permanently. Thus the development of the two types of individual seen in the Hydrozoa may be summarized as follows: Egg Blastula " Planula " Parenchymula Gastrula Actinula Polyp Medusa This development, though probably representing the See also:

PRIMITIVE

primitive sequence of events, is never actually found in its full extent, but is always abbreviated by omission or elimination of one or more of the stages. We have already seen that the parenchymula stage is passed over when the gastrulation is of the invaginate type. On the other hand, the parenchymula may develop directly into the actinula or even into the polyp, with suppression of the intervening steps.

Great apparent differences may also be brought about by variations in the See also:

period at which the embryo is set free as a larva, and since two free-swimming stages, planula and actinula, are unnecessary, one or other of them is always suppressed. A See also:

GOOD, JOHN MASON (1764-1827)

good example of this is seen in two common genera of See also:

BRITISH

British hydroids, Cordylophora and Tabularia. In Cordylophora the embryo is set free at the parenchymula stage as a planula which fixes itself and develops into a polyp, both gastrula and actinula stages being suppressed. In Tubularia, on the other hand, the parenchymula develops into an actinula within the maternal tissues, and is then set free, creeps about for a See also:

time, and after fixing itself, changes into a polyp; hence in this case the planula-stage, as a free larva, is entirely suppressed. The Hydrozoa may be defined, therefore, as Cnidaria in whit h two types of individual, the polyp and the medusa, maybe present, each type See also:

DEVELOPED

developed along divergent lines from the primitive actinula form. The polyp (hydropolyp) is of simple structure and never has an ectodernal oesophagus or mesenteries.' The See also:

general ectoderm loses its cilia, which persist only in the sensory cells, and it frequently secretes external protective or supporting structures. An internal mesogloeal skeleton is not found. The class is divisible into two main divisions or sub-classes, See also:

HYDROMEDUSAE

Hydromedusae and Scyphomedusae, of which See also:

DEFINITIONS

definitions and detailed systematic accounts will be found under these headings.

End of Article: HYDROZOA

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