g6 CRUSTACEA — CIRRIPEDIA chap. 



liiu'd externally and internally with chitin, opens anteriorly 



liy means of a circular aperture (oj)) guarded by a sphincter 



muscle. The visceral mass is composed chiefly of the two 



ovaries (ov), which open on either side of the meseuter}' by 



means of a pair of oviducts (ord) ; 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 witli the stalk or peduncle 



_.^^^-n.. > of a Lepas. The 



yff0^ y<\f/f'" -OV root-system passes out 



/^/ /^■■'i/f/Ji I fl through the ring of 



/■■.■^7 ■■■■■'■'^11^ attachment into the 



[■•■■'•■ '•■■■'■'#'■■• ~C I 111 l\ If-''- J- --' ■ A body of the host, and 



V-^'-''\ V-' •■■'■■■•■'••■ -^^ ff^ ramifies round the 



v'^^'^^^>-^^^''-'^'-^ organs of the crab ; 



^^"^^^<^v^^^^=:Tr'^^^ the roots are covered 



^'^^"^^■^-^}^-^:^{-^p,ji-^^:^ ^OVa externally with a thin 



<zi? chitinous investment, 



Fif!. 67. — Diagrammatic median longitiulinal section and COnsist of an epi- 

 througli a normal Cirripede. ^«, Brain ; lyj, mantle- ^.i i- i 



opening ; o'w/, oviduct ; (v/, vas deferens. tneilUm ami au in- 



ternal mass of branch- 

 ing cells continuous witli 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 Saccidina. The 

 young are hatched out in great numljers from the maternal 

 mantle-cavity as small JSTauplii (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. carciiii parasitic on Carcinus maenas at Plymouth and 



