KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 85 



distinction has been made evident by the present investigation. No other organs 

 present such distinct marks for discriminating the genera and no others correspond 

 so intimately to the shell characteristics than do the stomach and the nephridia. 

 That the hinge elements and the excretory organs in other groups of Lamellibranchs 

 show congruity in their phylogenetic development is a phenomenon to whicli I have 

 called attention in a previous treatise (1912) and which I have verified in a recently 

 published paper (1918); the present results are devoted to completely justifying this 

 view. 



Without entering into a discussion of the question rising from these statements 

 as to the reasons why the nephridia and the alimentary canal are, as it seems, 

 more adapted than other organs for establishing the relationship in the same direc- 

 tion as is pointed by the characteristics of the shell — for this embryological in- 

 vestigations are required which I hope to get an opportunity to carry out låter on 

 — I confine myself to drawing attention to the nature of these organs as inner ones 

 without any necessary predispositions, as it seems, of reciprocity to the exteriör 

 surroundings. Hence they grow less dependent of influences from without and can 

 more easily preserve an original construction than such organs as the foot, the 

 mantle and the gills. which show a more extensive faculty of adaption to various 

 external conditions of life. 



The configuration of the stomach and nephridia in Cltamidae thus supports the 

 separation of Pseudochama and Chama, but on the other hand their construction 

 gives evidence of a close mutual relation of these genera. Further they allow us 

 to judge, to a certain extent, about their affinity to other genera of heterodont 

 Lamellibranchs. 



Anthony (1905) arrives at the opinion that Chamidae are closely related to 

 Cardiidae: »11 me semble, en som me, que les Chames actuelles peuvent étre con- 

 sidérées comme provenant probablement de Cardiidae analogues par leur forme aux 

 Lithocardium, c'est-å-dire inéquilatéraux» (p. 326). 



If we prove this view with respect to the nephridium in Chamidae, we find 

 at once that the relative length of the pericardial tube is a characteristic not con- 

 fined to Cardiidae, where the pericardial tube is much shorter. This circumstance 

 speaks against the assumption of a close affinity in that direction. I pointed out 

 this difference in 1912 without entering into a more detailed investigation of the 

 matter, which was outside the scope of my work and apparently of secondary im- 

 portance. — As far as the stomach is concerned it has in Cardiidae no coecal appen- 

 dage such as is characteristic of Chama. 



If taken alone the organs in question would perhaps be less convincing evi- 

 dence for the necessity of a separation of the two familiae. But a closer investiga- 

 tion will reveal a great many anatomical points in which Chamidae and Cardiidae 

 differ considerably. Even the external division of the body is highly dissimilar: on 

 the one hand we have Cardiidae with a powerful foot and with large labial palps 

 attached at the inferior end of the anterior adductor, on the other hand Chamidae 

 with a strongly reduced foot and with large labial palps attached at the superior 



