96 CRUSTACEA—CIRRIPEDIA CHAP. 
lined externally and internally with chitin, opens anteriorly 
by means of a circular aperture (op) guarded by a sphincter 
muscle. The visceral mass is composed chiefly of the two 
ovaries (ov), which open on either side of the mesentery by 
means of a pair of oviducts (ovd); the paired testes (¢) are 
small tubes lying posteriorly in the mesentery, and the nervous 
ganglion (gn) les in the mesentery between oviducts and 
mantle-opening. A comparison with the condition of a normal 
Cirripede (Fig. 67) shows us that the mesenterial surface of 
the parasite by which it is fixed corresponds to the dorsal 
surface of an ordinary Pedunculate Cirripede, and that the 
ring of attachment corresponds with the stalk or peduncle 
of a  Lepas. The 
root-system passes out 
through the ring of 
attachment into the 
body of the host, and 
ramifies round the 
organs of the crab; 
the roots are covered 
externally with a thin 
chitinous investment, 
Fic. 67.—Diagrammatie median longitudinal section and consist of an epi- 
through a normal Cirripede. gn, Brain ; op, mantle- 
opening ; ovd, oviduct ; v/, vas deferens. 
thelium and an in- 
ternal mass of branch- 
ing cells continuous with the lacunar tissue in the visceral 
mass. 
The developmental history of the Rhizocephala is one of the 
most remarkable that embryology has hitherto revealed. It has 
been most accurately followed in the case of Sacculina. The 
young are hatched out in great numbers from the maternal 
mantle-cavity as small Naupli (Fig. 68, A) of a typical Cirripede 
nature, but without any alimentary canal. They swim near the 
surface of the sea, and become transformed into Cypris larvae of 
a typical character (Fig. 68, B). The Cypris larva, after a certain 
period of free existence, seeks out a crab and fixes itself by means 
of the hooks on its antennae to a hair on any part of the crab’s 
body. Various races of Sacculina are known which infest about 
fifty different species of crabs in various seas; the best known 
are S. carcini parasitic on Carcinus maenas at Plymouth and 
