ON STYLOCHUS PELAGICUS. 33 



digestive cells. These large cells are said by Gegenbaiir 

 to become converted into the skin glands. Figure 16 shows 

 the larva as seen in optical section. In the fore part of 

 the body a clear space intervened between the epidermis 

 and the large cells, which closely surround the digestive 

 tract in a complete layer. 



Two pairs of ganglia were seen connected by lateral com- 

 missures. The pair of auditory sacs were very conspicuous ; 

 one of them^ the left, was very much larger than the other. 

 They contained numerous small otoliths. Strong, longitu- 

 dinal, muscular bands were seen traversing the body, being 

 especially well marked towards its hinder extremity. No 

 ptera, suckers, nor hook sacs were found in the larvse, and 

 it is hardly possible that such existed, although Gegenbaur 

 draws attention to the great difficulty which is to be en- 

 countered in detecting the completely retracted ptera in 

 these larvae. 



The larva here described is evidently that of a Gymnoso- 

 matous pteropod. I cannot identify it exactly with any of 

 the numerous forms described or figured by Gegenbaur, 

 although it closely conforms to them in structure. It may 

 possibly be the larva of a Clione. It is figured here because 

 it seems of importance, since its anatomy was determined 

 with considerable exactness. The appearance of the median 

 part of the foot as two lobes instead of in a horse-shoe form 

 seems to be remarkable, also the imparity of the otolithic 

 sacs; but most remarkable of all is the advanced state of 

 development of the tissues and organs in a larva in which 

 the ptera have not yet begun to be formed. Dr. Hermann 

 Fol, in his elaborate memoir on the development of Ptero- 

 pods (' Etudes sur le Developpement des Mollusques, 

 Pteropods.' Paris: C. Reinwald, 1875), pp. 150, 151, 

 describes the asymmetrical development in the otocysts 

 of Pteropod larvae, and dwells on the importance of this 

 asymmetry as affording a means of estimating the amount of 

 asymmetry in the general growth of the two sides of the 

 larvae. 



He refers to the observed similar asymmetry in the deve- 

 lopment of the otoliths in a large number of Cephalophora. 

 In the Pteropod larvae observed by him the right otolith is 

 the larger, whereas in my larvae the left is the larger one. 

 Dr. Fol has never seen more than a single otolith present 

 in each otocyst during the larval existence of Pteropods, 

 although later on in development several small otoliths 

 became added to the primitive large otolith in each sac. 

 In my larvae no larger otoliths were present, the otocysts 



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