SACCULINA 



347 



of the pupa moults oft" ; the adult structures and habits are gradually 

 assumed. At frequent periods of continued growth the lining of the shell 

 and the cuticle of the legs are 

 shed. In spring these glassy 

 cast coats are exceedingly com- 

 mon in the sea. Acorn-shells 

 feed on small marine animals. 

 They fix themselves not to 

 rocks only, but also to shells, 

 floating objects, and even to 

 whales and other animals. 



On the ventral surface of the 

 abdomen of crabs, Saccidina, 

 one of the most degenerate of 

 all parasites, is often found. 

 Its history has been beauti- 

 fully worked out by Professor 

 Delage. It is in shape an ovoid 

 sac, and is attached about the 

 middle of a segment. On the 

 lower surface of the sac there is 

 a cloacal aperture, opening into 

 a large brood-chamber, usually 

 distended with eggs contained 

 in chitinous tubes. The brood- 

 chamber surrounds the central 

 " visceral mass," consisting of a 

 nerve ganglion, a cement gland 

 which secretes the egg-cases, and 

 the hermaphrodite reproductive 

 organs ; of digestive or vascular 

 systems there is no trace. The 

 parasite is attached by a pe- 

 duncle, dividing up into numer- 

 ous " roots," which ramify 

 within the body of the crab, 

 and by them the Sacculina ob- 

 tains nutrition and gets rid 

 of its waste products ; it is 

 practically an endoparasite. The 

 larvae leave the brood-chamber 

 as Nauplii ; they moult rapidly 

 and become Cyprid larvae. 

 These fix themselves by their 

 antennae to young crabs, at the 

 uncalcified membrane round the 

 base of large bristles. The 

 thorax and abdomen are cast 

 off ; the structures within the 

 head region contract ; eyes, 

 tendons, pigment, the remaining 



Fig. 191. — Development of Sac- 

 culina. — After Delage. (Not 

 drawn to scale.) 



A, Free-swimming Nauplius, with three 

 pairs of appendages ; B, pupa stage ; C, 

 adult protruding from the abdomen of a 

 crab. 



