130 DEVELOPMENT. 



eggs is evidently appropriated by the comparatively few embryos 

 which are thus developed at its expense. Prof. Carpenter 

 examined a large quantity of capsules, in which a considerable 

 number of small, free embrj'-os presented themselves before the 

 conglomeration of the great mass of the ova, so that he could not 

 doubt they were generated independently of it. The embrj^os 

 soon attach themselves to the conglomerate j^olk-mass, and by 

 the action of their cilia, the small segments of which it is 

 composed are driven down into their interior, which is soon 

 distended by them. The bodies which coalesce after segmen- 

 tation, Prof. Carpenter regards as imperfectl}^ fertilized ova, and 

 they evidently supplement the insufficient supply of nutriment 

 contained in the yolk-sack of each developing embryo. A similar 

 consumption of a portion of the ova takes place in Buccinum 

 and Nassa and very probably in a large portion of the proso- 

 branchiates. 



Generally, the shell and operculum developed within the egg- 

 capsule are retained by the animal, forming simply the nucleus 

 of the adult structure, but in a few cases a temporary shell and 

 operculum are provided, which are eventually lost. Animals in 

 this larval condition were formerly described as distinct genera 

 of pelagic gastropods, until Krohn, and after him Macdonald,* 

 showed their true relationship ; in this the lingual dentition 

 became an important agent to indicate the connection with adult 

 forms. Krohn discovered at Messina a curious mollusk which 

 he called Echinospira (xx, 49-50), and which proved to be the 

 larval state of Marsenia conspicua. He found the nucleus of 

 the permanent shell to be developed within the spiny nautiloid 

 larval shell, and that the latter was eventvmlly cast off. I figure 

 some other pelagic larval mollusks : Macgillivraj^a, with its 

 curious six- or eight-lobed tentaculiform velum (xx, 44-46), which 

 is the larva of Dolium, and Cheletropis = Sinusigera (xx, 47-48) 

 which, on account of its dentition has been (probably errone- 

 ously) referred to the Muricidse ; f but in all the egg-capsules 

 of Murex which I have examined the contained shells are 

 miniatures of the adults. Mr. Arthur Adams has referred a 

 Cheletropis to Purpura biserialis, and it is just possible that 

 the species belongs to the Purpurinse. Investigations of the 

 transformations of free swimming larvse are made with difticulty, 

 and it will probably be many years before we shall have 

 acquired a sufficient body of facts to understand the conditions 



* Maodonald, On Metamorphosis of Mollusca, Linn. Trans, xxii, 241 ; 

 xxiii, 69. 



f Chiropteron semilunare, Sars {Beskriv. og Jagtta gelser), t. 14, f. 38, 

 1885), is probably the larva of Aporrhais. Morch, Ann. Mag. iV". Hist., 

 3d ser., xvi, 78, 1865. 



