IN THE ANIMAL KINGDOM. 08 



testinal blood-vessel. At the point of connection the 

 vessel is perforated, and the lower part of the sac so intro- 

 verted on itself, that the blood can penetrate freely into the 

 intus-suscepted portion. In the upper part of the sac are 

 found spermatic and ovarian cysts, which discharge their 

 contents when mature into the main sac. After fecundation 

 from fifteen to twenty ova become invested with a common 

 capsule. The embryos have not yet been traced to full ma- 

 turity, but the course of development indicates a relation- 

 ship to the Pectinibranchiata, and, as Miiller thinks, to the 

 genus Natica. 



These facts do not seem explicable by any process of 

 alternation on the part of the Synapta — even were we pre- 

 pared to admit the existence of such a relation between an 

 Echinoderm and a Mollusc — for this is not a case of gem- 

 mation but of sexual generation ; and yet it is not the true 

 sexual generation of the Synapta, for this animal has the 

 proper reproductive organs and embryogeny of its own class, 

 and such organs have been found even in the same indivi- 

 dual with the Molluscan brood. Parasitism affords a more 

 plausible explanation than either "alternation" or the notion 

 of " heterogony," which at one time suggested itself to the 

 discoverer ; but even this view involves some startling ad- 

 missions, for we must regard the sac either as a retrogres- 

 sive development of a normal Mollusc, resulting in a degree 

 of degradation — " a vermiform metamorphosis," to use 

 Miiller's own expression — unparalleled even among the 

 Ctrrhipedes or Lerneans — or else as an alternating form, 

 budded off from a normal mollusc, for which we have as 

 yet no precedent among the Mollusca proper. We have 

 besides the difficulty of explaining the access of the para- 

 site, and still more the very intimate nature of its connec- 

 tion with the vascular system of its host, though these are 

 points which we can quite as little account for in some 

 other well-known cases, unquestionably of a parasitic nature. 



