DEVELOPMENT. 131 



under which a large portion of the prosobranchiates undergo 

 this larval transformation after exclusion, whilst in other genera 

 the newborn mollusk is the epitome of the adult. 



In the larval Chiton the velum is replaced b}^ a crown of cilia, 

 and eyes are present although the adult is blind. The shell, 

 when it appears, is the miniature of the mature form. 



In the pulmonata the velum is rudimentary, represented by a 

 ciliated tuft on each side of the mouth. In Limax there exists 

 on the dorsal posterior extremity of the body a pedunculated 

 vesicle, the contractions of which, alternating with those of the 

 vitelline sac, produce a sort of circulation which diminishes and 

 disappears as the heart is formed. The embryonic pulmonata 

 possess primitive kidneys or Woljian Bodies, consisting of 

 paired tubes, curved, without communication one with the other, 

 and the walls of which are provided with secretory cells. These 

 organs are formed hy a small invagination of the ectoderm. 

 The embryonic shell of the pulmonates constitutes the apex of 

 the adult shell ; where it is usually distinguishable by smoothness 

 of surface, hyaline and colorless appearance. 



Pteropoda (xx, 53). Most of these in the larval state resemble 

 those of the marine gastropods. They have a ciliated velum 

 and an operculated shell. The velum atrophies on the develop- 

 ment of the wings. The gymnosomate pteropods (Pneumodermon, 

 Clio), pass through a stage during which they are provided with 

 three girdles of cilia, and resemble the annelid embryos — ^as do 

 those also of the Dentalium. 



Scwphoj)oda (xx, 54, 55). The larval Dentalium is provided 

 with several circles of cilia. Its shell is constituted at first of 

 two symmetrical portions, with margins in contact; these finally 

 unite to form the single tube of the tooth-shell, an elongated 

 cylinder open at each end. 



Pelecypoda (xxi). The young bivalves are hatched before 

 they leave their parent ; the oviducts open into the dorsal 

 channels, and the eggs are received into the gill-tubes and there 

 retained until the escape of the young mollusk. In Unio and 

 Anodonta (xviii, 11), the outer gills only receive the eggs, with 

 which they are completely distended in the winter months. In 

 Sphffirium the inner gills form the marsupium, containing from 

 ten to twenty fry : some of these remain until nearl}^ a fourth 

 the length of their parent. 



The forms which youug bivalves pass through, present distinct 

 differences in several families, so that even in the present state 

 of embryological knowledge, some five or six types of develop- 

 ment are known. Even in the same family there may be a great 

 dissimilarit}^, as in the case of the marine and fresh-water forms 

 of the Mytilidse. The following account refers to the type to 

 which the young of Crenella belong. At first they have a swim- 



