54 



mouth. 



The Minute Structure of the Gills. - It has been 

 seen that, by their mode of formation, the "intra-f ilamen- 

 tar imion" between the tv/o limbs of a lamina is so com- 

 plete, that blood may flow freely through the lamina from 

 the afferent to the efferent branchial vein. The"inter- 

 filamentar" connection, between adjacent laminae is also 

 very complete, but serves only for support and not for the 

 full interchange of blood. The general plan of the inter- 

 laminar connections is shown in fig. 39, which is a tan- 

 gential section of a gill alnost in the line of the let- 

 ters iX»J.* i^ fig* 38. It is seen that the points of union 

 in adjacent laminae are arranged in regular rows. At each 

 point, the supporting rod (s.r. fig, 40) projects through a 

 perforation, so as to bind together adjacent laminae. At- 

 tached to adjacent rods are fiber-like cells which are ap- 

 parently muscular, and contractile. 



The minute stinxcture of the edges of a lamina is 

 similar to that of the filaments in forms like Mytilus, 

 though the various types of cells are more sharply marked 

 off from each other. At eith'^r side there are two rows of 



