MOLLUSCA. 117 



of the mantle and of a gill (ctenidium) on each side, composed of 

 an inner and an outer gill-plate, each of these being again made 

 up of an outer and inner lamella united together ventrally. The 

 outer lamella of the outer gill-plate is attached by its dorsal edge 

 to the inner side of the mantle, close to where it joins the body- 

 wall. The inner lamella of the outer gill is united above with 

 the outer lamella of the inner gill, and the common union of the 

 two (gill-axis) is attached above to the wall of the body. The 

 inner lamella of the inner gill-plate is attached anteriorly to 

 the body-wall, is then free for a short distance, and behind 

 the visceral mass unites with its fellow in the middle line to 

 form the floor of the cloacal chamber. The gill-lamellae are 

 vertically striated, and between the striations numerous minute 

 apertures are present, leading into an interlamellar space present 

 in the interior of each plate, which is divided into a number of 

 vertical water-tubes by the union of the two lamellae along a 

 corresponding number of narrow vertical strips by interlamellar 

 junctions. The visceral mass divides the anterior part of the 

 supra-branchial chamber into right and left halves. Each of these, 

 owing to the dorsal attachment of the gill-axis, is again subdivided 

 into two. There are, therefore, four channels above the gill- 

 plates in this region, into which the four sets of water-tubes 

 open. Behind the visceral mass the two inner channels coalesce 

 with one another, and, finally, in the cloacal region, the supra- 

 branchial chamber is undivided. These various relations will be 

 understood by examining Fig. 34. 



In Fig. 33 the cohering inner plates have been separated so as to expose 

 part of the supra-branchial chamber. 



The gills of Anodon are unusually complicated. They appear to have 

 arisen in the following way : From the gill-axis on either side two longi- 

 tudinal rows of filaments grew out. The ends of the outer row turned 

 sharply outwards and upwards ; those of the inner row inwards and 

 upwards. Thus in cross-section a W-shaped outline would be seen. These 

 filaments then fused into two folded lamellae, numerous apertures being 

 left, however. The descending and ascending halves of each lamella then 

 became connected to form a gill-plate. Union of the plates with one 

 another and with surrounding parts increased the complexity, and brought 

 about the distinction between branchial and supra-branchial chambers. 



Each gill is covered by a single layer of ciliated epithelium. 

 The external ridges (representing gill-filaments), which appear to 

 the naked eye as vertical striae, are supported by slender horny 



