STRUCTURE OF THE LAMELLIBRANCHIATE GELL. 299 



The gill in such genera as Area, Mytilus, and Modiola 

 is about midway between the series of separate tentacles 

 of the Cyclas embryo and the continuous lamella of Unio 

 and Anodon, and enables us to understand how the latter 

 is formed by the union of a row of tentacles. The com- 

 mon marine Mussel, Mytilus edulis, may be found in 

 abundance attached by its byssus to piles and rocks near 

 low tide mark. The general form of the gills may be 

 studied in living or alcoholic specimens, but for making 

 sections to show the minute structure, the gills should be 

 carefully removed from the body and placed for twelve 

 hours in a three-tenths of one per cent solution of chromic 

 acid, and then transferred to seventy per cent alcohol ; after 

 they have remained in this for a day or two they may be 

 transferred to strong alcohol, ninety per cent, and kept 

 until they are wanted. 



a. In an alcoholic specimen which has been carefully 

 opened note that, as in Unio or Anodon, there is an inner 

 and an outer gill upon each side of the body, and each 

 gill consists, as in Unio, of an inner and an outer 

 lamella. 



1. As in Unio, the inner lamella (Fig. 150,6) of the 

 outer gill, and the outer lamella (c) of the inner gill are 

 united dorsally to each other and to the body wall. 



2. The thickened ridge (z), formed by their union, con- 

 tains a blood-vessel (A 1 ). 



3. The outer lamella (a) of the outer gill, and the 

 inner lamella (d) of the inner gill, unlike those of Unio, 

 are free dorsally and end above in thickened ridges, which 

 also contain blood-tubes (&'). 



b. In a perfectly fresh living specimen, or in an alco- 

 holic specimen which has been carefully preserved 'and 

 opened, the surface of the gill is a broad, flat, vertically 



