PTEROPODA AND GASTROPODA. 569 



continue in almost their primitive form and proportions, unless the 

 broad lobes of the adult be substituted for the embryonal vela, as is 

 the case with the fins of the Pteropods ; otherwise the little Cymbulia 

 with its delicate symmetrical shell would represent a persistent em- 

 bryo form of the higher Gastropodous Encephala, Prof. Muller* 

 has detected ova and embryos of a Gastropod, which he believes to 

 belong to a species of Natica, within the body of the Synapta digi- 

 tata : they were contained in elongated sacs, firmly attached or fused 

 at one end to the head, at the other end to the gut of the Synapta. 

 The upper portion of the sac contains both spermatozoa (like those 

 of Natica) and ova ; the lower portion of the sac was intus-suscepted 

 with a blind end, and this contained the ova with developed embryos, 

 according to the velated type. This remarkable discovery indicates 

 some singular parasitic habit in the generative economy of the 

 moUusk (ante, p. 221.). 



The development of the pulmonated Gastropods proceeds without 

 any such metamorphosis as that above described. In the testaceous 

 species its course has been ably traced by Prevost and Dumortierf 

 in Limjiceus, by Pfeiffer J in Helix, and by Jacquemin and Quatre- 

 fages in Planorbis. § 



The transparency of the albuminous capsules of the ova, and of the 

 ova themselves in the fresh-water Pulmonata, renders them beautiful 

 and favourable objects for such researches. 



In the Physa, the nidamental mass is short and ovate: in the 

 Limnceus it is oblong, and not striated, as in the Planorbis. The 

 double movement of the embryo is more conspicuous in the LimncBus 

 than in the Planorbis. The first movement of the yolk is one of 

 rotation upon its axis ; but, as development proceeds and the ciliary 

 vibrations are strengthened, the embryo begins to travel in an ellip- 

 tical course around the interior of the Qgg ; its two movements (to 

 compare small things with great) resembling those of the planets in 

 the solar system. 



In the Planorbis the single centre of the ovum, or the germinal 

 vesicle with its nucleus, is very evident in the ovarian ovum. The 

 processes which lead to the disappearance of the vesicle, take place 

 in the oviducal pouch called the " matrix." The transparent nida- 

 mentum in which the ova are excluded is shield-shaped and striated ; 

 it is not attached to any foreign body but falls to the bottom. After 

 the usual subdivisions of the yolk, a group of less opake cells makes 

 its appearance in a particular part of the periphery of the granular 

 mass ; and an epithelial membrane begins to spread over its surface, 



* CLXX. t CCCLXVI. X CCCLXXV. § CCCLXXVI. 



