130 



THE CRUSTACEA 



of structure throughout the group. After a careful discussion of 

 all their chd-racters, Hansen considers that they must in all'proba- 

 bility belong to the Apoda, and he considers that the material 

 examined by him includes at least ten species. 



Morphology of Rhizocephala. 



The Rhizocephala are an exclusively parasitic group, nearly 

 all infesting Decapod Crustacea, and are distinguished from the 

 normal Cirripedes by the complete loss in the adult state of all 



r,;^SiUj^, 



mes 



Pig. so. 



A, SacciUina carclni in position on the detached abdomen of the crab ; one side of the mantle 

 has been removed, iiit, intestine of the host, surrounded by the roots of the parasite ; vi, 

 mantle ; iiics, mesentery ; 0, opening of mantle-cavity ; ov, egg-masses in mantle-cavity ; p, 

 peduncle ; v.m, visceral mass ; ? , opening of genital atrium ; the outline of the coUeteric 

 gland is seen surrounding it. The outline of the testis is seen abo^'e, a little to the left of the 

 jjeduncle. B, vertical section of Scwadina at right angles to the plane of the mesentery 

 (semi-diagrammatic); at, genital atrium; g, nerve-ganglion; gl, coUeteric gland opening into 

 genital atrium ; oi-^i, ovary ; r, absorptive roots ; t, testis ; other letters as in A. (After Delage.) 



traces of segmentation and of appendages, and (excluding for the 

 present the doubtful SjjJiaerothylacus) by the absence at all stages of 

 life of an alimentary canal. 



The body (Fig. 80, A) has the form of a simjile .sac attached 

 by a short peduncle, from Avhich root-like processes ramify through- 

 out the body of the host. These absorptive roots appear to be 

 absent in the aberrant genus Duplorhis. The visceral mass, or body 

 proper, is completely enveloped by the mantle, which has a narrow 

 aperture (c) capable of being closed by a sphincter muscle. In 

 Sylon the opening is double, and in Clistosaccus and Dnplm'his the 

 mantle-cavity is completely closed. The mantle is attached to the 



