318 EMBRYOLOGY 



of the body cavitj. Sej^raental organs have not. been 

 identified, though the oviducts were held by Beakd to be 

 remnants of such, and Nansen believed the same of the 

 paired ciliated depressions of the outer surface of the body 

 formerly called sucking discs ; but up to the present time 

 su tficient grounds for this view have not been produced . The 

 sexual organs in Myzostoma are not always developed in the 

 same way. In addition to the hermaphroditic individuals, 

 there are living on them very much smaller male animals 

 (complemental males). The fact that oviducts were also 

 found in these (Nansen) indicates that we have to do, not 

 with individuals of really separate sexes (Beard), but only 

 with incompletely developed hermaphrodites. 



The place which we assign to the genus Myzostoma appears 

 to be justified by the manner of its development. This 

 characterizes it as a branch of the Annelid stem, which, to 

 be sure, is T-ather aberrant, and has suffered great changes, 

 probably as the result of the parasitic mode of life. The 

 place pi'eviously ascribed to it, supplementary to the class of 

 Arthropods, was necessarily given up when the development 

 became better known. The form and internal organization 

 of the larva, as well as its ciliation, which is also a feature 

 of the adult animal, separate it shai-ply from the Arthro- 

 poda. 



V. HIRUDINEA. 



The Hirudinea, like the Oliyochieta, lay their eggs in 

 CQcoons, which are formed in the same way in the two 

 cases, namely, by a secretion from dermal glands, which 

 hardens. The cocoons themselves ai-e of various sizes, 

 according to the size of the animal. In the medicinal leech 

 they become more than 2 cm. in length. Their shape also 

 varies in different species and genera. Those of Uirudo and 

 Aulastoma are ellipsoidal, and exhibit outside the shell 

 proper a layer of spongy substance, which probably serves to 

 protect them against desiccation (Leuckart). They are 

 deposited in the earth. The flattened cocoons of Clepsine 

 and Nephelis are found in water, firmly glued to some fixed 

 object. Clepsine covers the cocoon with its body, and 



