KUNGL. SV. VET. AKADEMIENS HANDLINGAR BAND 59. N:0 3. 73 



Grieser gives a careful account of the composition and the histological struc- 

 ture of the alimentary canal in Ch. pellucida, so that it seems superfluous to re- 

 capitulate these conditions and I refer the reader to his vvork for them. Only some 

 points are added concerning the general morphology of the alimentary canal. Except 

 for the case described by Grieser there is no fact to support the opinion as to 

 inversion in the position of the organs of Chama and Pseudochama. The alimentary 

 canal, on the contrary, presents the same scheme in both genera. I have found no 

 support for the opinion expressed by Pélseneer (1911, p. 58) in these words: 

 »LMntestine s'y enfonce dans la partie droite de la masse viscérale chez ces Chama 

 'inverses', tand is qu'il s'enfonce a gauche dans les Chama gauches.> In reality the 

 intestine seems to tend to form a tura towards the right side in dextral (»inverse») 

 as well as in sinistral (»normal») forms of Chamidae. 



Even if one were to pro ve any cases of »inversion» of the alimentary canal 

 in any species of Chamidae, this might perhaps be explained from the conditions 

 prevailing in Area. Matthias (1914) has found that the duodenum emanates in 

 some cases from the left, in others from the right of the stomach, and that its 

 narrow furrow is always turned to the median line of the body, thus either to the 

 left or to the right. In such cases there can be no question of any inversion, but 

 only of a tendency towards orientation in a certain direction (tropism). 



4. The nervous system. 



In both Chama and Pseudochama the closest conformity prevails in the main 

 strueture of the nervous system. This is described in so complete a manner b}'' 

 Grieser (1913) that only some points as to the differences deduceable need be given 

 here. Grieser also discusses the peculiarities of the nervous system comparatively, 

 as far as this is possible with our present knowledge. The presence of a special 

 buccal centre as well as of a »median commissure» between the cerebro-visceral con- 

 nectives are peculiarities detected by Grieser in Chama pellucida, and I have found 

 them prevailing in all forms examined both of Chama and Pseudochama. 



In all the Chamidae, though they are of a sessile habit, there exist statocysts 

 situated somewhat above the pedal ganglia, each containing one statolith. 



5. The nephridia. 



In 1912 I gave a short note and a schematical figure of the nephridium of 

 Chama sp. (= Ch. sarda Reeve), showing its position behind the pericardium and 

 round the foot retractors, as well as the relatively large length of their pericardial 

 tubes in com parison with Cardium and Tridacna. A closer examination of the ne- 

 phridia was carried out by Grieser (1913) in Ch. pellucida where he found the same 

 conditions with respect to the position and the strueture. From his figures and 

 description it is evident that in Ch. pellucida the pericardial tubes are surrounded 

 by the distal sacs, these covering the former even on their median sides by lobes 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 10 



