378 



COMPARATIVE ANATOMY. 



way, e.g. in Chreseis 



pericardial orifice (>•") into the interior of the tube, where it opens 

 (r), so that it only communicates with the exterior in a round- 

 about manner. 



In the Thecosomatous Pteropoda, and also in the Heteropoda, the 



organ which is regarded as the 

 kidney, because of the similarity 

 between its two orifices and 

 those of the Prosobranchiata, is 

 also remarkable for its spongy 

 character. In Carinaria, among 

 the Heteropoda, it is provided 

 with a distinct investment of 

 secreting cells ; in all the rest 

 there is a clear cellular layer 

 instead. The framework of 

 the kidney is stiff; but in 

 Atlanta and the Firolida3 it is 

 contractile, and performs en- 

 ergetic and spasmodic contrac- 

 tions. In the Thecosomatous 

 Ptcropoda also, the kidney can 

 act in this 

 (Fig. 201, re). 



As the glandular nature of 

 this organ is doubtful when the 

 secreting cells do not contain 

 concretions, greater weight may 

 be laid on its relations to the in- 

 gestion of water, which has been 

 best observed in these cases. 

 The movements of the organ 

 are not limited to merely open- 

 ing and closing its external 

 orifice, but they also drive the 

 water inwards and mix it with 

 the blood which is returning to 

 the respiratory organs from the 

 general circulation ; this organ is 

 always placed in the course taken 

 by this current. Although the 

 ingestion of water by the excre- 

 tory organ has not been directly 

 observed except in these divi- 

 sions, there is no reason for 

 concluding that it does not also 

 obtain in the rest of the aquatic Gastropoda. It is in the Nephro- 

 pneusta only that the pai'ts concerned have other relations, but even in 

 them the kidney has the same relations to the system of blood-canals, 

 for the fluid of the blood maybe observed to pass out by the renal orifice. 



Fig. 201. Organisation of Chreseis. 

 P)> The fins, ce (Esophagus, v Stomach. 

 r Hind-gut. //Liver, a Auricle, c Ven- 

 tricle, re Kidney, x Its internal orifice. 

 / External orifice. h Ciliated organ. 

 (j Hermaphrodite gland, g' Hermaphro- 

 dite duct. </' Penial pouch, m Points to 

 the position of the retractor muscle. 



