. Elements of Comparative Anatomy. 378 COMPAEATIVE ANATOMY. pericardial orifice (r") into the interior of the tiibe, where it opens ()•'), 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 Prosobranchiataj 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 inst
. Elements of Comparative Anatomy. 378 COMPAEATIVE ANATOMY. pericardial orifice (r") into the interior of the tiibe, where it opens ()•'), 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 Prosobranchiataj 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 Firolidfe it is contractile, and performs en- ergetic and spasmodic contrac- tions. In the Thecosomatous Pteropoda also, the kidney can act in this way, in Chreseis (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 parts concerned have other relations, but even in them the kidney has the same relations to the system of blood-canals, for thefluidof the blood maybe observed to pass out by the renal Fig. 201. Organisatiou cf Chreseis. jip The fins. 03 (Esophagus, v Stomach, r Hind-gut. ?i Liver, a Auricle, c Ven- tricle, re Kidney, x Its internal orifice. x' Exter
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