Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/681

CRUSTACEA G47 been investigated by numerous observers. De Geer, Rathke, 1 Spence Bate, 2 and Anton Dohrn 3 have made careful observations on the embryology and development of this abundant freshwater Isopod. As in Ligia the embryo is bent upwards within the egg. It quits the egg in a most imperfect state, more so, says Rathke, than any other articulated or vertebrated animal. It is furnished in its earliest stages with two lateral external appendages, which probably are homologous with the foliaceous ap pendage observed by Fritz Miiller at the back of the head in Ligia. These zoeal appendages are subsequently moulted. Moreover in the young Asellus there are only six leg-bearing segments, and six, instead of seven, pairs of legs as in the adult. The curvature of the embryo upwards, instead of downwards, seems to have been generally ob served by Rathke, Dohrn, Fritz Miiller, and others. The larval skin is in some genera so closely applied to the egg itself as possibly to be mistaken for an inner egg-membrane. The absence of the last pair of thoracic feet seems also a constant character ; all the other limbs are usually well developed in the young of normal Isopods ; but in the remark able and aberrant genus Tanais (fig. 38) all the abdominal feet are wanting, but not the caudal appendages; they make their appearance, however, simultaneously with the last pair of the thoracic feet. Among the many interesting facts relative to the develop ment of the Crustacea not the least remarkable are the series of retrograde metamorphoses which certain Isopods undergo as a consequence of their assuming a parasitic mode m FIG. 38. Tanais dubius (?) Kr. 9 . magnified 25 times, showing the orifice of entrance (x) into the cavity overarched by the carapace in which an appendage of the second pair of maxillae (/) plays. On four feet (i, k, I, m) are the rudiments of the lamellK which subsequently form the brood-cavity. (Fritz Mliller.) of life when adult. Thus the Cymothoa-, or " Fish-lice," which in the adult state live parasitic on fishes, clinging firmly by means of their short recurved hook-like feet, are FIG. 39. Bopyrus squil!arum } Latr. a, male; 6 , female (underside); 6, the same (dorsal view). (After Spence Bate.) fia.W.P/ti-t/xusabdominalis. a, the male; 6, female (ventral aspect). (After Spence Bate.) FIG. tl. Cryptothiria Balani. a, male t 6, female ; c, larva. (After Spence Bate.) lively free-swimming Crustacea in the larval state. Still greater is the metamorphosis which the adult female under- 1 Abhandlungen zur Bildungs imd Enlwickelungs Geschichte des Menschen und der Thiere (Leipsic, 1832). 2 Spence Bate and Westwood, Hist. Sessile-eyed Crustacea (1868, vol. ii. p. 346-347). 3 Die embryonale Entwickelurrg des Asellus aquaticus" (a reprint from Zeilsch. /. wissensch. Zoolojie, xvii. Bd. ii. Heft 1, Jena, 1866). goes in Bopyms (fig. 39), Phryxus (fig. 40), lone, Gyge, and several other allied genera, which are parasitic on crabs and lobsters, taking up their abode within the branchial cavity. The adult is usually quite destitute of eyes ; the antennae are rudimentary ; the broad and flat body is fre quently unsymmetrically developed in consequence of the confined space in which it lives ; its segments are more or less amalgamated together ; the feet are stunted, and the abdominal appendages transformed into foliaceous or highly branched gills. The males are diminutive in size, but usually they have their eyes, antennae, and feet better preserved than the females ; the abdomen is, however, rudi mentary, and not unfrequently altogether destitute of ap pendages. Among the Isopoda, in the remarkable genera Crypto thiria, Cryploniscus, and Entoniscus, we meet with forms even still more debased in their adult parasitic condition than Bopyrus, In the case of Cryptothiria Balani 4 (fig. 41), first noticed as a male cirripede by Goodsir in 1843, but not rightly determined until 1851, by C. Spence Bate, the female is a large inert seven-lobed fleshy mass, destitute of exserted antennae, jaws, legs, or branchial appendages, lying within the shell, and attached to the base of the animal of Balanrs balanoides. The male is free and resembles the male in the Bopyridce ; its body is long and slender, and is furnished with seven pairs of legs ; it has been met with by Spence Bate, Dana, and other observers within the body-cavity of Balani. Here then we have a Crustacean belonging to a higher order, viz., the Isopoda, living parasitic within the shell and deriving its nourishment from one belonging to a lower order, viz., the Cirripedia. The history of Cryptothiria pygmvea (Rathke, sp.) and Fig. 42. FIG. 42. Cryptoniscus planario ides, female. Magn. 3 times, (Fritz Miiller.) FIG. 43. Embryo of the same. Magn. 90 dium. (Fritz Miiller.) Cryploniscus planario ides (F. Miiller) is perhaps still more remarkable. Professor Bell had long ago noticed the frequent presence of a singular parasite on the inner surface of the abdomen of Portunus and Carcinus on our coasts, having prima facie the aspect of a bag of immature eggs. This had teen described by Rathke in 1841 as an Entozoarian, but has since been proved by its transforma tions to be a Cirripede, and was named Peltogaster. In 1858 Lilljeborg found what he deemed to be a female Peltogaster with an egg-sac but a careful dissection led to the discovery that another parasite of a higher order, namely a Cryptothiria, had become parasitic upon the parasite. The most curious part of this super-parasitic history is, that the roots of Sacculina and Peltogaster (two forms of rhizocephalous Cirripedia parasitic on crabs and hermit-crabs) seem constantly to be made use of by two parasitic Isopods, namely, a Bopyrus and the before-men tioned Cryptoniscus planario ides. These take up their abode beneath the Sacculina, and cause it to die away by inter cepting the nourishment conveyed by the roots ; the roots, however, continue to grow, even without the Sacculina, and frequently attain an extraordinary extension, especially when a Bopyrus obtains its nourishment from them ( Fritz Miiller, op. cit. p. 94). The free males and the young of Cryptothiria and Cryptoniscus are unlike young Cirripedes; 4 C. Spence Bate and J.O. Westwood, British Sessile-eyed Crustacea

(1868, 8vo. vol. ii. p. 267).