Cirripedia. 35 
Order 2. Rhizocephala. 
The Rhizocephala 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 has a small opening to the 
exterior. From the short stalk by which the animal is attached, 
Fig. 14. 
Sacculina carcini 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. 
Sacculina carcini , of which a specimen is exhibited (Fig. 14), 
is found on the common shore-crab ( Carcinus mamas) and other 
Crabs. 
The remarkable changes which the presence of Sacculina 
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. Drawings of the larval stages of Sacculina are exhibited. 
d 2 
