Cirripedia. 35 



Order 2.— Rhizocephala. 



The Ehizocephala are parasites living on other Crustacea, and Table-case 

 they offer one of the most striking examples of the degradation in No. 4. 

 structure associated with the parasitic habit of life. In the adult 

 they lose every trace, not only of Crustacean, but even of Arthro- 

 podous structure, although the very close resemblance of their 

 larval stages to those of the normal Cirripedes shows that they 

 have been derived from forms similar to the latter. The body is 

 enclosed in a fleshy mantle, which lias a small opening to the 

 exterior. From the short stalk by which the animal is attached, 



Fig. 14. 



Saccnlina carciiil attached under the abdomen of a common Shore-crab. 

 [Table-case No. 4.] 



fine root-like filaments branch in all directions throughout the 

 body of the host (generally a Crab), and serve for the absorption 

 of nourishment. The parasite has no mouth or food-canal, no 

 limbs, and only a feebly developed nervous system. 



Saccnlina carcini, of which a specimen is exhibited (Fig. 14), 

 is found on the common shore-cral) {Carcinus viaenas) and other 

 Crabs. 



The remarkable changes which the presence of Saccnlina 

 induces in its hosts are illustrated by a series of specimens in 

 Wall-case No. 2 already referred to. 



In their larval development the Rhizocephala pass through 

 Nauplius and Cypris stages closely similar to those of ordinary 

 barnacles. Drawmgs of the larval stages of Sacculina are exhibited. 



D 2 



