176 GASTROPODA. 



this subject (v. Jhering, No. 46 ; Simroth, No. 120).* Since the 

 I T eliger stage may still be found even among the undoubted Pulmon- 

 ates, although usually in a somewhat reduced condition, no object ion 

 can be made to this classification from the ontogenetic stand-point. 

 Their development, however, shows in an unmistakable manner that 

 we have to do with transitionary forms, a fact which is further con- 

 firmed by their manner of life, especially by that of Ondiidium 

 (p. 133). 



The velum, it should be mentioned, is, according to Semper, well 

 developed in some tropical forms (Auricula, Scarabu*; No. 118), in 

 the same way as in Onchidiam (R. Bergh, No. 5, p. 175). Semper 

 assumes that the larvae of these forms swim about freely in the 

 sea. Since they, as already stated, also possess an operculum, they 

 bear a great resemblance to the Opisthobranch larvae. As a rule, the 

 velum is much reduced in the Pulmonates. These pass through the 

 invagination-gastrula stage, the manner in which this gastrula arises 

 being modified in many ways according to the varying amount of the 

 yolk. Thus the archenteron, in consequence of being composed of 

 the large, yolkdaden cells, appears at first as a massive structure 

 with a narrow lumen, but at a later stage widens out and becomes 

 a spacious sac. The originally narrow cleavage-cavity also gradually 

 widens out. The embryo is now spherical. Its animal pole is often 

 marked by the presence of the polar bodies ; at the opposite vegetative 

 pole is found the blastopore, which at first is wide, but narrows later 

 and usually becomes slit-like. It closes from behind forward, but, 

 apparently, a small anterior aperture may remain. At this point, 

 in the midst of an ectodermal depression, the mouth forms, and, 

 when the blastopore is retained, it becomes displaced somewhat far 

 inward by the stomodaeum to the point at which the stomach com- 

 mences (Fol, No. 33 ; Rabl, No. 91 ; Wolfson, No. 131). 



The spherical or often somewhat ventrally flattened form of the 

 embryo undergoes some alteration in consequence of the appearance 

 of the shell-gland, the foot and the velum. The shell-gland arises 

 as an ectodermal invagination on the dorsal surface opposite the 

 mouth (Fig. 78). It may sink in so deep that it has repeatedly been 

 mistaken for the rudiment of the proctodaeum. It flattens out again 



* [Plate (Z<><>1. Jahrb. Anat., Bd. vii., 1891) who has recently made a thorough 

 study of the anatomy of Onchidium, concludes that while these forms are 

 ti.if Pulmonates, they nevertheless show affinities with the Tectibranchiate 

 Opisthobranchs. He places the Onchidiidae and Vaginulidae as direct 

 derivatives of the primitive pulmonate on a branch quite independent of the 

 Stvlommatophora or Basommatophora. — Ed.] 



