26 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



The gills. On the sides of the body two pairs of branchiae are present. In 

 their upper part they are unequal in size, due to the inequilateral shape of the 

 animal, but on both sides they entirely cover the body, extending from the mouth 

 to the posterior adductor. The branchial axis descends in a dorso-ventral direction 

 from the front lateral side of the pericard and is connected to the side of the body 

 in the vicinity of the visceral ganglion. Here it receives a nerve from this ganglion 

 and is then disconnected from the body and becomes freely hanging into the mantle 

 cavity ; at its lower end it is separated from the mantle as well as from the gill of 

 the opposite side. The branchial axis encloses, besides the blood vessel, a strong 

 longitudinal muscle. 



The gill filaments emerge from the axis in a forward and backward direction. 

 The considerably larger anterior or inner gill extends to the mouth; its chief part 

 hängs freety down into the mantle cavity and shows a plication in the direction of 

 the filaments, these being gathered in groups of about 13 each, separated by furrows; 

 about 10 such groups or plicae are present. 



Above the mouth the free f rontal margin of the gill, which is furnished with 

 a marginal furrow in its ventral edge, becomes attached to a limb that emerges 

 from the space above and between the two labial palps (fig. 23). In this limb, 

 along its inside, a large blood vessel runs in a dorsal direction up to the umbones 

 and across their upper and outer sides communicates with the branchial vein in- 

 cluded into the branchial axis. 



Both the outer and the inner lamella of the anterior gill is well developed, 

 above the mouth as well as below it. Even the inner one is plicated in a way just 

 corresponding to the outer one, so that the furrows meet each other and the boundary 

 filaments of the opposite lamellae are combined by vascularized bridges. Intra- 

 lamellar conjunctions between the filaments of adjacent plicae occur as well as close 

 intraplical ones throughout the whole gill, even in its umbonal part. 



The upper margin of the inner lamella is entirely free from the body, and it 

 is only high up, at the umbones, that it joins it. It contains a blood vessel and a 

 supporting cord of muscular or connective tissue, both corresponding to the same 

 features in the branchial axis and passing into it below. Finally on the umbones 

 the blood vessel of the branchial axis also receives that of the inner lameliar margin. 



The posterior gills likewise emerge from the branchial axis at their uppermost 

 end and first send their filaments forward to constitute a descending sheet, than, 

 after being reflected backwards at the edge of the gill, they form a reflected lamina 

 which is expanded posteriorly beyond the branchial axis as an appendix. covering 

 the pericard and the nephridia, as far as the posterior adductor. The filaments of 

 the outer lamina, which have run parallel to those of the inner sheet, make a sharp 

 curvaticn just outside the axis of the gill, where they pass into the »appendix» and 

 are here directed obliquely downwards. By the strong elevation of the right side 

 the filaments of the right gill describe a sharper curve, about a right angle, than 

 those of the left. A further difference between the opposite sides appears in a strong 

 projecting fold in the posterior part of the back gill and behind it a deep groove 



