182 



GASTROPODA. 



tion forms which at first is shallow but soon becomes deeper ; this 

 is the rudiment of the respiratory cavity which continues to widen 



and thus comes to include 

 md. v. 



ma^ 



aw. 



Fig. 80. — Older Planorbis embryo, seen from the 

 back (after Rabl). au, eye ; eg, cerebral ganglion ; 

 /, foot ; ma, edge of the mantle ; mrf, enteron 

 and digestive gland ; [>g, pedal ganglion ; s, shell ; 

 t, tentacle ; un, primitive kidney ; v, velum ; 

 vd, stomodaeum. 



the anus and the aper- 

 tures of the adult kidney. 

 This cavity itself opens 

 externally only through 

 a narrow aperture, the 

 respiratory aperture, 

 which lies rather far- 

 forward on the right 

 side of the body. 



The formation of the res- 

 piratory cavity has also been 

 viewed in another way ; viz., 

 as a fusion taking place 

 between the margin of the 

 mantle and the body, only 

 a small aperture being left, 

 wbich, as respiratory aperture, leads into the greatly deepening cavity. In 

 this way, the respiratory cavity is shown to be the transformed pallial or 

 branchial cavity. In the Basommatophora this is indisputable, as a gill 

 is in some forms found in the cavity (Amphibola). The respiratory cavity 

 in the Stylommatophora has, on the contrary, been regarded as not homo- 

 logous with the branchial cavity, but rather as the ureter transformed 

 into a respiratory organ. On this account v. Jhering termed the terrestrial 

 Pulmonates the Nephropneusta, thus distinguishing them from the aquatic 

 Pulmonates, which he named the Branchiopneusta (Nos. 45 and 46.) We 

 ourselves do not find anything in the manner of formation of the respira- 

 tory cavity in land Pulmonates to justify so different an interpretation of it. 

 The mantle-cavity in the Prosobranchia may also at first, as here, arise appar- 

 ently in the form of an ectodermal depression. The homology between the 

 respiratory cavity of the land Pulmonates and that of the water Pulmonates, 

 which in itself is so probable, is further supported by the fact that in some of 

 the former (Testacellidae, Plate, No. 89) a sensory organ is present in it which 

 corresponds to Spengel's olfactory organ found lying near the gills in the 

 mantle-cavity of other Gastropods. 



Towards the end of that period of embryonic life during which the 

 embryo may be compared with the larva of other Gastropods, the 

 sinuses in the head and the foot which gave rise to the embryonic 

 circulation above described undergo gradual degeneration. In the 

 same way the primitive kidney disappears and the permanent kidney 

 functions in its stead. 



The final form of the animal is reached by the growth of the parts 



