136 GASTROPODA. 



the greater part distributed in the form of isolated spindle-shaped 

 cells in the primary body-cavity. Its further development will be 

 described later, but we must here refer to one of the organs formed 

 from the mesoderm, the primitive kidney, since this is essentially a 

 larval organ. 



Each of the primitive kidneys arises from a compact mass of 

 mesoderm-cells, two such masses lying at the sides of the embryo 

 behind the velum. A lumen now appears in the mass, which, by 

 lengthening somewhat, becomes a short tube and, coming into con- 

 tact with the ectoderm, fuses with the latter and thus opens externally 

 not far behind the velum. The ectoderm sinks in somewhat at this 

 point later ; in Bythinia, this ectodermal invagination is even very 

 deep and forms the longer, distal part of the primitive kidney (v. 

 ERLANGER). The inner surface of the tube becomes covered with 

 cilia especially at the blind end. The primitive kidney remains short 

 in Paludina, but, in the Pulmonata, appears as a long bent tube. 

 This is said to possess an internal aperture, that is to say, it com- 

 municates with the (primary) body-cavity (p. 179): v. ERLANGER 

 was unable to convince himself of the presence of such an aperture in 

 Paludina and Bythinia* and, taking into consideration the condition 

 of the primitive kidneys in the Annelida, we may conclude that it is 

 wanting in these forms and that the two renal tubes end blindly. 

 This is certainly the case in the earlier stages. At the inner end of 

 each kidney, there is a bundle of spindle cells which in all cases 

 extend to the ectoderm, and serve for suspending the renal tube. 

 This latter attains its highest degree of development at the somewhat 

 advanced stage shown in Fig. 99, and degenerates later (BiJTSCHLi, 

 v. ERLANGER). 



The Trochophore form of the embryo is now specially modified by 

 the development of the foot on the ventral surface as a massive pro- 

 minence (Figs. 56 B and 57,/). The appearance and rapid increase 

 in size of this organ leads to a considerable displacement of the other 

 parts of the body (Figs. 56 B and 58). The pre-oral part of the body 

 becomes still more flattened out. The mouth shifts to the anterior, 

 end and the velum finally appears displaced to a dorsal position (Fig. 

 58). The oral aperture and the anus lie at the two opposite ends of 



* [v. ERLANGER (No. 71), however, describes an internal aperture in 

 Pulmonates. MEISSENHEIMER (Nos. XVII. and XVIII.) has made a most 

 careful investigation of this point in Limax and is firm in his belief that 

 there is no internal opening in that Pulmonate. He derives the entire 

 organ from the ectoderm. ED.] 



