Class 3. Acephala. 



807 



lu a few forms {e.g., tbe Oyster) it is altogether wanting ; in 

 others it is rudimentary or small. 



Each gill is a lamella, the upper edge of which is attached to 

 the body ; the lower portion, almost half the lamella, is bent round* 

 and closely apposed to the upper half (just as one half of a folded 

 sheet of paper lies upon the other) ; in the outer gill the reflexed 

 portion lies external to, in the inner gill internal to, the upper portion. 

 The edge of this reflexed portion is, in some cases, free, in others 

 it is concrescent with the body close to the point of origin of the 

 gill lamella, either throughout its whole extent or for part of its 

 length only. The gill lamella consists of delicate filaments extending 

 dorso-ventrally ; they are sometimes free, only adhering to one 

 another by "ciliated junctions" {Mytilus), but are usually firmly 

 united by interfilamentar junctions, so that the gill lamella shows a 

 trellis-work. The reflexed portion of the gill lamella is usually 

 united with the upper portion by similar " ciliated junctions " (Pig- 

 252). On the surface of the gill lamellae is a thick covering of 

 cilia, by which the water is kept circulating through the meshes, and 

 small, solid bodies are conducted towards the mouth by means of 

 the ciliary currents on the surfaces of the gills. 



In some Lamellibranohs (Fig. 2bSA), each gill lamella projects 

 behind the foot in a free point ; the 

 two points of the same side have, 

 however, undergone concrescence 

 along their dorsal edges.t In others 

 the point has become concrescent at 

 its hindmost tip, with the posterior 

 part of the mantle (Mussels). In 

 others again (Fig. 253 B), the right 

 and. left inner portions of the gills 

 coalesce behind the foot along 

 their reflexed edges, and similarly 

 the outer gills with the mantle ; in 

 this case, in the posterior region of 

 the mantle-cavity, an upper portion 

 of the cavity (suprabranchial) is 

 separated from the rest, and extends 

 into the cloaca. 



The mantle is divided into 

 two symmetrical portions, a right 



and a left mantle-lobe, which lie external to the gills. Bach 

 lobe is a thin lamina, the edge of which, the pallial muscle, 



* Only in a few Lamellibranohs are the gills not reflexed. 



t The two gill lamellae of each side, in all probability, together represent a plumose 

 gill with two series of filaments ; sometimes the two lamellae originate from a common 

 basis, 



X 2 



Fig. 253. TransTerse sections through 

 the posterior regions of two LameUi- 

 branchs after the removal of the shells, 

 to show the relations of the hinder 

 ends of the gills, kr body, Tc 

 mantle, g inner, g^ outer giU, h supra- 

 branchial portion of the mantle cavity. 

 — Orig. 



