EECENT WOEKS ON THE ENTOZOA. 335 



should find their way to the body of a second (perhaps a thn\l or 

 fourth) 'bearer.' In some instances this second animal-host is nearly 

 allied to the first, diifering from it, however, in habit ; but often it 

 belongs to a separate class, or even sub-kingdom. 



Bearing in mind that these changes are by no means of the same 

 simple character as those of ordinary metamorphosis, we may adopt 

 the very convenient terminology of Van Beneden, and arrange the 

 life cycle of the agamogenetic Scolecida under four periods, viz. \- — 

 («.) The ' proscolex,' or embryo. 



(h.) The ' scolex,' or sexless zo5id developed therefrom. 

 (c.) The * strobila,' or mature organism, with 

 {d.) The ' proglottis,' or generative zooid. 

 Earely is the second of these stages apparently wanting. Much 

 more frequently do these organisms persist in the sexless condition 

 for an indefinite period of time, continuing to grow, and multiplying by 

 internal gemmation. In this manner many successions of ' nursing- 

 forms,' as they have been termed, may arise from the development of 

 a single embryo. 



The structure of the Scolecida is scarcely less interesting than 

 their development. It would be impossible, within our present limits, 

 fully to discuss the problems which the study of their numerous 

 anatomical relations suggests. In the body of most Scolecids 

 we may, however, easily recognise the existence of the following 

 principal systems of parts : — 



(a.) A digestive system, 

 (b.) A tegumentary and connective,* 

 {c.) A reproductive, 

 {d.) A nervous, and 

 {e.) A pseudo-vascular system. 

 In two orders, Cestoidea and Acanthocephala, a digestive system 

 is wholly wanting. Among the Trematoda the alimentary canal 

 takes the form of a straight or branching tube, having but one aper- 

 ture, the oral. The Gordiacea, quoad their digestive system, come 

 between the Trematoda and anenterous Entozoa; that singular 

 genus, SphcBruIaria, having an alimentary apparatus almost as 

 simple as that of the Infusoria. The Nematoidea^ in spite of their 

 parasitic mode of life, have a digestive system not very different from 

 that of the Annelida. 



* Under this head, the ' muscular system ' might also be described. 



