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THYROSTRACA
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THYROSTRACA , an See also:order of See also:Crustacea, comprising barnacles, See also:acorn shells and some allied degenerate parasites. The embryos are See also:free-See also:swimming, active forms, but in adult See also:life the animals are fixed See also:head downwards, and are very degenerate. The See also:body is indistinctly segmented, and is enveloped in a See also:fold of the integument, usually with calcareous plates. The anterior antennae are fused with the anchoring See also:attachment, whilst the posterior pair is vestigial, and the appendages of the mouth and body See also:present various degrees of degeneration and specialization. In most cases the adults are hermaphrodite, but unisexual forms also occur, whilst the hermaphrodite adults may carry with them See also:minute " complementary " See also:males. In strong contrast with the See also:condition in most Crustacea, the spermatozoa are See also:mobile. As shown by Burmeister in his See also:historical See also:review (1834), these animals, comprised by See also:Linnaeus in the genus Lepas, first received a more comprehensive See also:title from See also:Cuvier, who called them Cirrhopoda, a word strictly meaning tawny-footed. See also:Lamarck in ',Soo altered this into the hybrid See also:form Cirrhipoda, meaning curl-footed, which was subsequently improved into Cirripedia or Cirrhipedia. So See also:long as the See also:group was held to be a subordinate member of the See also:Entomostraca, this See also:term, though not the earliest, was generally accepted. The name Thyrostraca, meaning doorshells or See also:valve-shells, is preferred as agreeing in termination with the titles of the other two divisions, the See also:Malacostraca and Entomostraca. The group may conveniently be arranged in two See also:principal sections—the Genuina with cirrhiform feet, and the Anomala without them. Thyrostraca genuina.—It is with these that See also:Darwin's classical See also:treatises (See also:Ray See also:Soc. and Palaeont. Soc., 1851-1854) are almost exclusively concerned. Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single See also:species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia. Within the last Kochlorine (Noll, 1872) with two species, and Lithoglyptes (Aurivillius, 1892) with three species, have since been included. But H. J. See also:Hansen (See also:Die Cirripedien der See also:Plankton-Expedition, 1899) states that Cryptophialus minutus, for which the order Abdominalia was founded, has, like Alcippe and other Genuina, its cirrhi on the See also:thorax, not, as Darwin wrongly supposed, on the See also:abdomen. In See also:place, therefore, of the Abdominalia, it will be right to accept the See also:family Cryptophialidae, v. See also:Martens, See also:side by side with the Lithoglyptidae of Aurivillius and the Alcippidae of Gerstaecker. These, with Darwin's three families above mentioned, See also:complete the See also:section of genuine cirripedes now existing. Gruvel submitted to the Linnaean Society a rearrangement of the Lepadidae, unfortunately using for the first of his new families the preoccupied name Anaspidae. It is See also:con-fusing, but not uninstructive, to find that within the Balanid group such generic titles as Stephanolepas and Platylepas have been coined. The See also:vernacular name See also:barnacle, traceable to the See also:fable of pedunculate cirripedes hatching out into bernicle geese, has also been transferred to the sessile cirripedes, which are popularly known as acorn barnacles. A complete See also:list of all the See also:recent species of Thyrostraca in both sections, down to the See also:year 1897, was published by Weltner (See also:Arch. Naturg., 1898, § 63, pt. i. pp. 227-280). For fossil species, See also:Woodward's See also:Catalogue of Brit. See also:Foss. Crust. (1877), pp. 137-144, should be consulted. Hoek (" Challenger " Reports, Cirripedia," 1883, viii. 8-n), gives a brief mouth." Darwin notes B. improvisus as quite tolerant of See also:water See also:geological See also:summary down to 1882. In that year J. M. See also: Aurivillius considered that Pollicipes signatus showed a closer approach to the Balanidae than any other of the Lepadidae, but he, too, in See also:ignorance of the Devonian Protobalanus (Whitf.), discoursed needlessly about the gap in the See also:distribution. Dr Bather justifiably anticipates further discoveries, but if, already in Silurian as in See also:modern times, the members of these families had to pass through nauplius and cypris stages to maturity, there is one " enormous gap " between them and the See also:common ancestor of the crustacean class that will not be easily filled. To later phylogenetic links an addition is offered by Dr See also:Wood-See also: The cirripede, which has an elastic peduncle, a crested capitulum, but no valves, and the first cirrhi longer than the See also:rest, should stand near Eremolepas, the name given by Weltner in place of the preoccupied Gymnolepas (Aurivillius). In the genus Scalpellum, S. giganteum, Gruvel (Trans. Linn. Soc., 1901) disputes with S. stearnsii (fig. 2), Pilsbury, 189o, which shall be accounted the greater. The latter is threatened with a new generic name (Chun, Aus den Tiefen See also:des Weltmeeres, 1900, p. 502). The See also:horizontal distribution of barnacles over all seas is fully explained by Darwin. The bathymetric range of sessile as well as pedunculate forms down to such depths as twelve or eighteen thousand feet—Verruca quadrangularis, Hoek, 1900 fathoms; Scalpellum See also:regium, Wyville See also:Thomson, 285o s ii. (Contribution a l'etude des Cirrhipedes, 1894) found that the species frequenting See also:sea sur- See also:face or shallow water, notwithstanding their feeble See also:powers of See also:vision, cannot live long when entirely debarred from See also:light. It must, therefore, be supposed that abyssal forms have gradually acquired such tolerance of darkness as makes their See also:health See also:independent of the See also:sun. Among other singularities of See also:habitat, not the least curious is the freedom with which some small species, especially in the genus Dichelaspis, occupy the very jaws of large crustaceans. It is generally stated that cirripedes are confined to See also:salt water, and, generally speaking, that is true. But Platylepas bissexlobata (De See also:Blainville), from the See also:west See also:coast of See also:Africa, is sometimes found entirely buried, except its operculum, in the skin of the manatee. Now, since it seems this Manatus senegalensis ascends See also:rivers, we may infer that its See also:parasite travels with it. See also:Studer (Crustacea of the Gazelle, 1882) records Balanus See also:amphitrite (Darwin?) from roots and stems of mangroves in the See also:Congo, where, he says, " it follows the mangroves as far as their vegetation extends along the stream, to six sea-See also:miles from the not saline. Why the Thyrostraca, so See also:hardy, so widely dispersed and multitudinous, and with a See also:history so prolonged, should not have made more extended and more independent incursions into fresh water remains a problem. Though the Ornitholepas australis (Targioni Tozzetti, 1872), found on the tail feathers of a See also:bird, represents only the cypris-larva of a cirripede, it still shows one of the many facilities for See also:dispersion which these creatures enjoy. A striking instance of their abundance is cited by Aurivillius (1894) from a See also:report by See also:Captain G. C. Eckman, who See also:late in the summer observed See also:great masses of Lepas fascicularis forming broad belts in the See also:North Sea. Aurivillius himself examined a humpback See also:whale which had as many as fifty specimens of Coronula diadema on each side of its head. He believes that the cetacean approaches not only rocks, but See also:ships, in the See also:hope of freeing itself from its lodgers. Yet the fact that the long, soft Conchoderma auritum stands exposed on the Coronula, sometimes ten on one, indicates that the whale can have little See also:chance of evicting its tenants, even at the expense of rubbing off the eighteen flattened horns of its own skin embedded in cavities See also:round the domed See also:base of the Coronula shell. The fecundity in the genus Lepas has struck many observers. Hoek (" Challenger " Reports, Cirripedia," 1884, vol. x.) notes that, while in Scalpellum the number of eggs may be less than a See also:hundred, " in Lepas anatifera it amounts, on the contrary, to many thousands and tens of thousands." In the same See also:treatise Dr Hoek has useful chapters on the See also:anatomy, development and sexes of the group, with references to the important researches since Darwin by Krohn, Claus, Kossmann and others. See also:Francis Darwin, in the life of his See also:father (1888, iii. 2), says, " Krohn stated that the structures described by my father as ovaries were in reality salivary glands, also that the oviduct runs down to the orifice described in the Mono-graph of the Ciryipedia as the auditory meatus." Hoek, however, observes that the See also:interpretation of the glands as salivary is not given by Krohn as his own See also:opinion, but only quoted from Cuvier. Hoek himself proposes to See also:call them pancreatic glands. On the absorbing question of the development, T. T. See also:Groom (Phil. Trans., 1894, vol. clxxxv.) supplies a full bibliography, beginning with the pioneers Slabber (1778; properly 1769) and J. See also:Vaughan Thomson (183o). Groom's monograph was almost immediately supplemented by Chun's chapters on the same subject (Bibliotheca Zoologica, 1895, Heft 19, Lieferung 2), to which an important discussion is contributed by H. J. Hansen (Die Cirripedien der Plankton-Exp., 1899). He insists on the value of the upper See also:lip or See also:labrum for generic distinction, and as an aid in affiliating larval forms of different stages to their several species. He cites Groom's See also:evidence that larvae obtained from the See also:egg readily go through one See also:moult in the See also:aquarium, and the known fact that the last larval See also:stage is marked by a See also:longitudinal series of six pairs of immovable spines or processes. He considers, then, that by a judicious comparison of larval forms with these two easily determinable stages the poverty of existing See also:information on the subject may be gradually, if laboriously, diminished. The large and See also:peculiar Archizoea gigas of Dohrn must, he thinks, belong to the Lepadidae as a larva in the last stage, but not, as v. Willemoes Suhm supposed, to Lepas australis, or even to the genus Lepas at all.
Thyrostraca anomala.—This section comprises Darwin's Apoda, the footless, Lilljeborg's Suctoria, called by Fritz See also: 53) argues that various nauplii of a type not previously described may probably be referred to this group or family. The second family, discussed by Delage, Giard, Kossmann and others, has no dearth of genera and species, though about several of them the in-formation is scanty. Almost all of them are parasitic on other crustaceans. Sphaerothylacus polycarpus (Sluiter, 1884) has an ascidian for its See also:host. Sarcotaces (Olsson, 1872) has two species parasitic in fishes. But these exceptional and dubious forms do not obtain nutriment by sending rootlets in a rhizocephalous manner into their patrons. The family Peltogastridae is sometimes separated from the Sacculinidae, and sometimes made to do See also:duty for both, the latter course being improper, since Sacculina (J. Vaughan See also:Thompson, 1836) is not, as has been supposed, preoccupied, and must, therefore, take See also:precedence of Peltogaster (Rathke, 1843). In the same family stands the genus Sylon, noted by Kroyer with-out a name in 1842, named by him without a description in 1855, described by See also:Michael Sars in 1869, and published by G. O. Sars in 187o. Hoek (" Challenger " Reports, 1888, vol. xxiv. app. A) will orientate the See also:English reader on this genus. For the complicated See also:parasitism of isopods and Sacculinidae on the same hosts Giard and See also:Bonnier (Bopyriens, 1887, p. 197) should be consulted. The remaining family may, till further knowledge, be allowed to See also:cover four remarkable species, three of them See also:resident on See also:Anthozoa, one on an echinoderm. Only the first, the celebrated Laura gerardiae (Lacaze-Duthiers, 1865), sends such rootlets into its host as would justify the term Rhizothoracida. The small sinuous segmented body is enclosed, except for one small opening, in an enormous sac-like See also:carapace, between the lamellae of which are protruded from the body the ovary and " See also:liver," both large, bifurcate and ramified. It is this sac-like and not valvular carapace, therefore, that justifies the term Ascothoracida. But Synagoga mira, See also:Norman, 1888 (Brit. Assoc. Report for 1887), has the body covered by two nearly circular valves instead of a sac. Petrarca bathyactidis, G. H. See also:Fowler (Quart. Journ. Mic. Sci., 1889, vol. See also:xxx. pt. ii. p. 115), has a bilobed carapace, ventrally open; Dendrogaster astericola, Knipovitch (Biologisches Centralblatt, 1891, X. 707), is a multilobular sac, with apparently indistinct segmentation of the body proper on the dorsal side. For this highly problematic group the See also:original authorities should by all means be consulted. The student may then be asked to compare the See also:account of Synagoga mira both with the figure of the cyprisstage of Dendrogaster astericola and with the figure of the " indeterminate See also:animal found on Gerardia," about which Lacaze-Duthiers asks, " Is it the cypris-stage of Laura?" (Mem. Acad. Sci., 1883, xlii. 16o, pl. 1, fig. 102). S. mira was found, like Laura gerardiae (fig. 3), in the Mediterranean, and found like it attached show the body proper. to an antipatharian. Its six pairs of limbs are not like the See also:bare and See also:simple feet of the Laura, but two-branched and setose as in the See also:ordinary cypris-stage of the cirripede. The conclusion, therefore, from these facts and from the suggested comparisons, seems at least extremely probable that the question asked by H. Lacaze-Duthiers should be answered in the affirmative, and that S. mira is either the cypris-stage of Laura gerardiae or of some congeneric species. In Lacaze-Duthiers's highly-elaborated memoir it should be noticed that he uses the term " cirrhes " rather misleadingly, not for cirrhiform feet, but as the See also:equivalent of setae. Also he gives two different reckonings of the segmentation, counting first eleven body segments without the caudal furca (p. 40), and then the caudal furca as itself the See also:eleventh segment (p. 41). Of Petrarca the development is not yet known. The points of agreement and difference between it and Laura are carefully See also:drawn out in the See also:essay by Dr G. H. Fowler, who inclines to favour a See also:close relation-See also:ship between the Thyrostraca and Ostracoda. To the extreme development of the carapace in Laura, as compared with the segmented body, it would be difficult to find among crustaceans any See also:analogy more striking than that of the great ovarial expansions in Nicothoe astaci, the little copepod parasite of the common See also:lobster. The compactness of the class Crustacea is generally admitted; of the precise See also:affinities of its subdivisions there is still much to learn. (T. R. R. Additional information and CommentsThere are no comments yet for this article.
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