M. E. Claiiaiedc un the Decelupmetit of Neritiua flu\ iatilis. 199 



of the still inoperculate foot. At this period the internal organs 

 of the abdouiiual part or true body are not recognizable. The 

 velum very soon increases rapidly, becoming developed like a 

 hem, whilst its margin acquires longer, more distinct, and more 

 numerous cilia. This margin grows considerably thicker in 

 proportion to the membrane of the velum. In the abdomen a 

 spacious cavity is formed, and in this a quantity of forma- 

 tive mass collects as an aggregation of fatty drops of various 

 sizes. The foot forms an oval organ possessing thick walls and 

 an inner cavity which seems to be in connexion with that of 

 the abdomen. The foot is still completely destitute of cilia. 

 The pit-like anterior impression becomes converted into a trans- 

 verse buccal orifice, which leads into a tubular ojsophagus. The 

 entrance into the mouth is ciliated all round, as is the whole 

 surface of the oesophagus. The same thing was seen by other 

 observers in the embryos of many Mollusca. 



Thus the alimentary canal makes its appearance in the em- 

 bryo of Neritiua as soon as the foot, or perhaps sooner. In 

 Actaon, according to Vogt, the alimentary organs are very 

 late in appearing, — much later, for example, than the otolithes 

 and the shell. The same was observed by Sars, Loven, &c., 

 especially in the Nudibranchiata ; and, according to Koren and 

 Danielssen, the formation of the heart in Buccinum precedes 

 that of the oesophagus, although this appears improbable, both 

 from Carpenter's statements with regard to Purpura, and from 

 what takes place in Neritiua. In Neritiua the oesophagus is 

 formed and lined with cilia before the first traces of the auditory 

 and visual organs, the shell, &c., have appeared; and Carpenter 

 met with the same conditions in Purpura. Leydig also says 

 that the embryos of Paludina are furnished with a mouth and 

 anus, and the foundations of the pharynx and intestine, before 

 any trace of an ear exists. 



This early appearance of the alimentary organs is of great im- 

 portance for the further development of the Neritina. From 

 this moment it is no longer a mere immature embryo, but a 

 larva, swimming freely about in the capsule, and feeding upon 

 the remaining contents of the latter, the sister-eggs, which have 

 not arrived at development. This is not a mere assertion, ren- 

 dered probable by the gradual increase of the minute embryo 

 and the simultaneous disappearance of the rest of the yelk-mass, 

 until the embryo fills the whole capsule, — but a fact proved by 

 observation. If the capsule be opened carefully, the little em- 

 bryo escapes and swims about amongst the numerous groups of 

 globules already mentioned as formed by the breaking-up of the 

 abortive eggs, which are perfectly transparent and of a pale 

 golden-yellow colour ; they consist of a homogeneous, tenacious, 



