KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3. 49 



Nephridia. The left nephridium is the highest one, reaching to the upper 

 wall of the rectum. The pericardial tubes emerge from the lateral corners of the 

 pericard; behind the short oseular tubes they expand into compressed ampullae with 

 folded walls, which are quite embedded in the outer säcs by means of a pair of 

 median lobes descending on their inner sides. The pericardial tubes are in their 

 upper parts enclosed in the back wall of the outer sacs. These are slightly lobate 

 all round, mostly in their frontal and under sides and lined with a simple layer of 

 bladder-like cells. The nephroprocts are situated close beneath and outside the 

 nephrostomes and opposite to and below the genital papillae. 



Chama reflexa forma jukesi Reeve. 

 Figs. 56—64. 



A large series of specimens from the north-western coast of Australia, collected 

 by Dr. Mjöberg, has been the subject of the following investigation. This ample 

 material shows a complete transition of the external characteristics (sculpture and 

 colour) into the larger Ch. reflexa, for which reason I have considered the latter 

 species as the full-grown and Ch. jukesi proper as the young, though already mature, 

 form of one single species, viz. Ch. reflexa. This view (expressed in my work of 

 1917, p. 29) is proved to be maintainable even on the basis of the inner organization. 



External morphology (fig. 56). 



The mantle is thin throughout or comparatively slightly thickened towards 

 the umbones, which are generally small and project equally on both sides. When 

 the left umbo is strongly developed, as a result of the stronger concavity of the left 

 valve, the mantle on this side is somewhat more thickened than usual. The si- 

 phonal openings are situated in the hindmost third of the lower margin. The anal 

 siphon is small, about */* of the length of the posterior adductor, and has no or only 

 a very weak siphonal fold. The terminal margin of the siphon is smooth, its basal 

 one may be minutely papillated ; there are no or only very few pigment döts round 

 the siphon. The length of the pedal slit is about V* of that of the anterior adductor. 



The labial palps occupy a length of about V* — V? of the anterior adductor; 

 they are fixed at about the same level and the left pair is directed downwards, the 

 right one backwards. 



The adductors are almost equal in size, the anterior one only a little larger, 

 and both are shorter than the distance between the upper ends of the adductors. 

 Further, the anterior adductor is in its upper part usually narrower than the inser- 

 tion length of the palps, and the posterior one is broader than the posterior gill. 



The foot is narrow, lengthened, its length being about 4 times its breadth 

 and equal to or larger than 1 U of the anterior adductor. Its form, however, changes 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 7 



