96 CRUSTACEA -CIRRIPEDIA CHAP. 



lined externally and internally with chitin, opens anteriorly 

 by means of a circular aperture (0/>) guarded by a sphincter 

 muscle. The visceral mass is composed chiefly of the t\v<> 

 ovaries (ov), which open on either side of the mesentery by 

 means of a pair of oviducts (ovd) ; the paired testes (t) are 

 small tubes lying posteriorly in the mesentery, and the nervous 

 ganglion (gn) lies 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, 



Flo. 67. Diagrammatic median longitudinal section and Consist of ail epi- 

 through a normal Cirripede, gn, Brain ; op, mantle- f i i- i 



opening ; ovd, oviduct ; r,l, vas defereus. 



ternal mass of branch T 



ing cells continuous with the lacunar tissue in the visceral 

 mass. 



The developmental history of the Ehizocephala 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 Nauplii (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 



