CO NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



Externa] morphology (fig. 72). 



The mantle is thin even on the umbones, which are situated at almost the 

 same height. In the under margin the two siphonal openings occupy less than half 

 but more than a third of the length from anterior to posterior adductor. Beneath 

 the expiratory opening a strong siphonal fold projects inwards. The pedal slit is 

 comparatively large, equalling in length about half the anterior adductor; above it, 

 on the frontal side of the adductor, the mantle is open. 



The adductor muscles are of about equal size or the anterior one is slightly 

 larger; the latter is shorter than the interadductoral distance. 



The labial palps are fixed at about an equal height on both sides, the ones 

 on the left side, however, being somewhat higlier; they are very small, their length 

 being scarcely V 7 °f that of the anterior adductor. The ones on the left side have 

 their tips directed backwards. The opposite pairs are united by thin rim-like lips. 



The foot is much compressed from the sides and has the shape of a some- 

 what quadrangular disk; its length is equal to or slightly exceeds its breadth and 

 is about l U of the length of the anterior adductor. 



The gills cover the body almost entirely; the front margin of the anterior 

 pair is curved or sinuated, leaving the labial palps uncovered. The posterior gills 

 are of about the same breadth as the anterior ones. The ascending lamina of the 

 rear gills has its margin fused with the mantle in front of the adductor. The inner 

 lamina of the anterior gill is free above, but is attached to the body along its lower 

 half, symmetrically on both sides. With their lower parts both pairs of the gills 

 are fused with each other and to a septum or a triangulär lobe on the hind part 

 of the visceral pouch, as well as to a siphonal fold which is formed by and projects 

 from the mantle. These structural conditions cause the presence of a closed branch- 

 ial chamber. In the other and larger specimen from the same locality all the cases 

 of coalescence mentioned are less pronounced and perfect. The posterior gills are 

 thus free from the mantle, even at their lower ends; the anterior ones are for their 

 most part free from the body and only attached at the extreme back to the tri- 

 angulär lobe and to each other, and their tips are separated from each other and 

 freely projecting within the siphonal fold. 



Internal anatomy. 



Alimentary canal (fig. 73). The oesophagus, immediately above the mouth, 

 shows in a transverse section a dorsal furrow and a ventral list as well as 5 lateral 

 f urrows on each side, the second of which is larger than the others. The two largest 

 opposite furrows are situated obliquely to the median line, the right one somewhat 

 higher up. 



At its upper end the length of the stomach is somewhat greater than its 

 breadth. From its left cardiac side a corner projects towards the front; it is, at 



