158 GASTROPODA. 



side, a position which is constant in forms like Atlanta, in which the 

 operculum is retained throughout life. The operculum, however, as 

 well as the shell, is frequently thrown off during the metamorphosis 

 (Firoloida, Pterotrachea). 



While these changes have been taking place in the foot, the velum 

 has gradually attained its highest development and then commences 

 to degenerate. The rudiments of the tentacles have already appeared 

 on the velar area ; these arise, curiously enough, quite asym- 

 metrically. One tentacle only is present at first (Fig. 66, t). At 

 the bases of the tentacles, the eyes appear. In some forms, the 

 tentacles may be reduced again {Pterotrachea). Before the shell is 

 thrown off, the velum has for the most part degenerated, only a few 

 traces of it being still found near the eyes. 



The body increases greatly in length, not only on account of the 

 development of the caudal section just described, but also through 

 the extension of the anterior part (the development of the so-called 

 proboscis, Fig. 68). In consequence of the great lengthening of the 

 foot and the anterior part of the body, the visceral sac lies, as in 

 other Gastropods, on the upper side of the body (Fig. QS). At the 

 junction of the visceral sac and the dorsal wall of the anterior part 

 of the body, the mantle-cavity has arisen and the gill has formed. 

 Where the shell is retained, it covers the visceral sac (Carinaria, 

 Fig. 68), and in Atlanta, which also as an adult possesses a shell, the 

 whole animal can still be withdrawn into it. 



C. Opisthobranchia. 



The ontogeny of various forms of the Opisthobranchia has been 

 studied by many zoologists. The embryonic development and the 

 younger stages of the free-swimming larvae were those usually in- 

 vestigated, the animals being difficult or even impossible to keep in 

 confinement during the later stages. Saks (Nos. 104 and 105) and 

 Loven (No. 69) established the chief features of their ontogeny, 

 while at a later period Adler and Hancock (No. 1), Nordmann 

 (No. 80), Vogt (No. 127), Schultze (No. 113), and Keferstein 

 (Nos. 52 and 53), occupied themselves principally with the develop- 

 ment of the larval forms and of the shape of the body. Bay Lan- 

 kester (Lamell. Lit., No. 29), Trinchese (No. 125), Blochmann 

 (No. 8), Bho (No. 93), turned their attention also to the internal 

 processes, especially to the earliest ontogenetic stages. References 



