Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/700

666 CKUSTACEA (2) ABDOMINALIA. The single Cirripede, the Crypto- phialus minutns, forming this section is the smallest known, being less than -j^-th of an inch in length. It is met with imbedded in vast numbers in the living shell of Con- cholepas Peruviana, the crypts almost touching each other. The three abdominal somites bear three pairs of cirri, the thoracic somites baing apodal. The sexes are distinct, the minute almost globular male being lodged within the crypt occupied by the female. Darwin found from one or two up to seven males attached to the same female. (3) APODA. This section, like the last, contains only a single form, the Proteolepas bivinda, resembling the larva or maggot of a fly attached by two threads ; the mouth is suctorial ; it has no limbs ; its body is shell-less, and it lies parasitic within the sack of Alepas cornuta. Cirripedia occur attached to the most varied objects, living and dead, throughout the seas of the globe. Sessile forms like Tubicinella, Coronula, Platylepas, Chelonobia, &c., are found attached to, and imbedded in, the epidermis of the whale, and on the sliell of the turtle, &c. Peduncu- lated forms are similarly widely distributed. Fritz Miiller calls attention to one anomalous form described by Darwin, ths Anelasm% squalicola, parasitic upon sharks in the North Sea, which seems to offer a remarkable analogy to the Rhizocephala. This Lepadide, he says, seems in a fair way to lose its cirri and buccal organs altogether. " The widely- cleft shell-less test is supported upon a thick peduncle, which is imbedded in the skin of the shark. The surface of the peduncle is beset with much-ramified hollow filaments, which penetrate the shark s flesh like roots." Cement glands and cement were not visible. " It seems to me," says Fritz Miiller, " hardly doubtful that the ramified hollow filamants are themselves nothing but the cement ducts converted into nutritive roots, and that it is in consequence of the development of this new source of nourishment that the cirri and buccal organs are in the highest degree aborted." All the mouth organs are minute and rudimentary ; the cirri thick, inarticulate, and desti tute of bristles ; the muscular tissue without transverse Btriation, and the stomach perfectly empty. 1 "The Lepadidce," writes Darwin, "include a much greater range of forms than the BcUanidce, and this is what might Lave been expected, for it is the most ancient family, and extinction has done its work in separating genera which, according to analogy, were once more nearly connected by intermediate forms." The most ancient sessile Cirripede found fossil is the Pyrgomi cretacea, H. Woodw., from the Chalk. Previous to 1865 the oldest-known pedunculated Cirripede was the Pollicipes rkceticus, Moore. In that year the writer described a curious aud most anomalous form of Cirripede, from the Upper Silurian of Dudley, with imbricated calcareous plates (fig. 86), under the name of Turrilepas Wrightii, previously described as a Chiton. The fossil form with which it has bsen compared is more perfect and equally bizarre, viz., the Loricula pulchella (see 6 in fig. 70), originally discovered in the Chalk of Rochester and since in that near Norwich. It affords evidence of a most aber rant form of Lepadide, in which the capitulum is very snull; the tody of the animal was lodged in a broadly- expanded peduncle, clothed in five rows of smooth lori cate tl calcareous scales, which, if toth sides were perfect, would have possessed ten rows and the plates would have GS which lives concealed, holding tightly to the under side of flat stones at low water, does exactly the same thing with its maxillipeds as the b .rnacles do with their cirri ; it keeps up a semicircular sweeping movement, so that a constant current conveys all the small living and dead objects within reach of its mouth. 1 Fritz Mu ller, op. cit. p. 140, and Darwin, op. cit. p, 170 PI 4 fizs. 1-7. exceeded 200 in number. In Bate and Westwood s Sessile- eyed Crustacea (vol. ii. p. 268) is figured a larval form of Cryptothiria Balani (reproduced in fig. 41 above), which FIG. 8G.TurriJepas Wrightii, II. Woodw. (Chiton VTrightii,T>& Kon.); U.Silurian, Dudley. The detached figures, a, b, c, indicate the three forms of plates of which the peduncles are composed in 1,2, and 3 which bear the corresponding letters. The opercular valves are not known. seems to afford evidence of a similar arrangement of plates. Possibly Loricula was parasitic like Bopyms. In conclusion, sufficient evidence has been adduced to show that the normal development of the Crustacean class has been one of progressive advancement, the forms of to-day, when viewed as a whole, being more highly developed and more differentiated than those which the geological record has preserved to us. But in any large community or class it is only the few that outstrip the many in the struggle for existence. Thus the Podoph- thalmia and Edriophthalmia present numerou3 examples of high advancement, both in intelligence and in attaining to a terrestrial life, especially in the Dec.ipoda-Brachyura. The Ostracoda, Phyllopoda, and Xiphosura are good instances of merely persistent forms. They are orders the members of which have branched out long since into by ways of their own, where, being checked from further progress, they have, by their great tenacity of life and large powers of reproduction, held their ground through the long lapse of ages from Silurian times to the present day, whilst higher orders have been modified or swept away. But the history of some Crustacea has been retrogressive, probably in a few instances from arrested larval develop ment, as, for example, in the case of the imperfectly- developed fifth pair of legs in Porcellana, Galathea, and Munida ; in most instances, however, retrogression seems clearly traceable to the parasitic or sedentary mode of life which the members have adopted. We have examples of this in the loss of eyesight in Crustacea passing their lives in subterranean caverns, wells, and streams ; the loss and atrophy of a part of the defensive armature of the body in the burrowing Thalassinidte ; the complete loss of the abdominal calcareous covering in Pac/urus ; the general atrophy of limbs and loss of symmetry of the body in the Bopyridw through residence within the branchial chambers of other Crustacea ; and the complete or partial loss of all or nearly all recognizable Crustacean characters in the adult female, or in both sexes, by parasitism on Crus tacea, fishes, &c., in certain of the Bopi/ridce, in the Cope- poda, in the Rhizocephala, and in the Cirripedia. Viewed as a whole, the Crustacea probably offer the best illustration of a class constructed on a common type, retain ing its general characteristics, but capable of endless modi fications of its parts, so as to suit the extreme requirements of every separate species. The outline of this great class here attempted necessarily lacks many important details ; these must be filled in by the reader from the various works referred to throughout

the article. (si. w.)