tains a ratfter large blood vessel that is connected with the ves- 

 sels of the large and the small filaments, and thus serves to 

 distribute blood either from or to the large vessels of the large 

 filaments, which are connected in their turn to the vessels that 

 supply blood to or take blood from the gills. 



As the free edges of the gills are approached the inter-fila- 

 mentar junctions become less and less prominent until, near the 

 margin, the filaments near the middle of the water tubes are 

 connected only by bunches of cilia like those in the mussel, 

 Mytilus, and some other forms. That the junctions of this 

 region should be less prominent is what might be expected, for 

 the margins of the gills, after the gills have become sufficiently 

 developed to show adult structure, are the growing and conse- 

 quently the youngest portions. That the filaments should at 

 first be connected by cilia only, may be looked upon as an indi- 

 cation of past history. The scallops presumably have had 

 ancestors in which the gill filaments were united by cilia only. 



Inasmuch as the gills are respiratory orgens, the arrangement 

 of the blood spaces in them is of more than ordinary interest. 



Two blood vessels are present in each of the suspensory mem- 

 branes of the gills, near the borders of the gills that it supports. 

 These vessels follow along the borders of the gills from near 

 their anterior to their posterior ends. One, the dorsal, that is 

 the one farthest away from the gills, supplies both of the gills 

 with blood, the other, the ventral, which is very near the borders 

 of the gills, receives the blood that is returned from both of the 

 gills. The blood enters each gill by branches from the supply- 

 ing vessel (figs. 17 and 18. ba'.) that run along- the upper bor- 

 ders of the inter-lamellar junctions to the edge of the free 

 lamella. Here they enter the large modified filaments of this 

 lamella (fig. 17, ba".) and are continued down to the margin 

 of the gill, giving ofif vessels to each of the inter-filamentar junc- 

 tions except those near the margin of the gill, which consist of 

 cilia only and are accordingly 'not vascular. 



Through the inter-filamentar junctions the blood is supplied 

 to the small filaments, so the blood vessels become a net work 

 that corresponds to the structure of the gill itself. The blood 

 makes its way around the margin of the gill, through somewhat 

 broken passages to the other lamella. This takes place all along 



27 



