THE CESTOIDEA 123 



marginal ; but in Ephedrocepkalus the male organ opens here, the 

 vagina on the ventral surface. The eggs undergo little change so 

 long as they remain in the uterus ; but, as in the Dibothridiata, 

 develop outside the parent, from which they are discharged by the 

 decay of the joint. The proglottids are detached singly, and the 

 fringe of processes or lobes which frequently ornament the hinder 

 margin are used as locomotor organs ; after separation the pro- 

 glottid may grow, and the genital organs undergo further changes, 

 so that there is a close resemblance to an " individual " organism 

 leading its own independent life. 



Very little is known about the life-history of the members of 

 the order. Van Beneden discovered isolated heads attached to 

 " sacs," or even enclosed in them, in various Selachians and other 

 fishes ; he noted the resemblance of these " scolices " to the heads 

 of various Tetraphyllidea, and rightly concluded that they represent 

 a stage in the life-history of the tapeworm. Such " metacestodes " 

 (which resemble Cysticercoids with everted head) occur free in the 

 intestine of various fish, and no doubt attain their adult state when 

 these are devoured as food by Selachians. One of the commonest 

 forms is Scolex polymorphus, which has been observed in a variety 

 of Teleosteans, in Sepia, and even in crabs. Monticelli (29) has 

 shown, by careful comparison with the head of adult Calliobothrium, 

 that it is the larva of C. filicolle, Zsch. In these metacestode 

 stages the organism consists of a sac and a head, invaginable 

 in some cases into the sac, the Avails of which are thin, mus- 

 cular, and provided with nerves and excretory canals, on the same 

 plan as in the adult ; the latter open by a contractile bladder 

 posteriorly, and are continued into the scolex anteriorly. By a 

 process of budding just behind the suckers, a series of segments 

 are formed, and it appears that the orginal sac or bladder upon 

 which six hooks have in many cases been recognised becomes the 

 most posterior, sterile proglottid. 



ORDER 2. Diphyllidea, v. Ben. 



The scolex is provided with a long " head stalk," which is armed 

 with several longitudinal rows of hooklets-; the "head" consists of a 

 retractile armed rostellum and four (apparently only two) phyllidia, with 

 projecting, slightly mobile margins. The strobila consists of few 

 proglottids. The generative organs are of the Tetraphyllidian type, but 

 the genital pore is on the ventral face. 



SOLE FAMILY. ECHINOBOTHRIDAE. Echinobothrium, v. Ben., sole genus ; 

 in spiral gut of Selachians (Fig. XVIII.) ; E. musteli, Pintner, the meta- 

 cestode in the liver of Nassa reticulata. The rostellum has an elaborate 

 structure, consisting of a muscular mass lying dorsal ly and ventrally, 

 supporting two groups of " frontal hooks." Tins rostellum appears to be 

 homologous with that of the Taeniidae rather than with the proboscis 



