supported along their sides by extensive inter-filamentar junc- 

 tions as is commonly the case, is evident, but I have not 

 examined other forms in which the filaments are similar, to find 

 if similar braces exist. 



The epithelial cells on the outsides of the filaments bear 

 numerous rather short cilia (fig. 21, fc.) that have to do with 

 moving currents over the surface of the gill. Between the 

 filaments, but near their outer borders, the epithilial cells are 

 modified in shape so they collectively give rise to a strong band 

 of cilia (oc.) on each side of each filament. In transverse 

 sections of filaments each of these bands appears as a bunch. 

 These cilia are concerned in forcing water through the ostia and 

 thus in creating the currents of water that furnish the food and 

 oxygen for the animal. 



The large modified filaments are roughly triangular in cross 

 section and like the smaller filaments each has a surface layer 

 of epithelium. The epithelium on the outer surface of the fila- 

 ment is thickly covered with cilia that correspond to the surface 

 cilia of the other filaments but there are no bands of cilia along 

 the sides. Inside the layer of epithelium is a pair of chitinous 

 rods (fig. 21, cr.) that run nearly to the free margin of the gill. 

 These rods are elastic and quite stiff and serve to keep the gill 

 in shape. Similar rods are present in each filament in many 

 forms but there seems to be no sign of them in the scallop in 

 any but these enlarged filaments. Considerable connective 

 tissue and well developed bands of muscle (mf.) are present, 

 that together cut up the large blood space (bv'. into a number 

 of small ones which are, however, connected with each other at 

 frequent intervals, so they may be regarded collectively as one 

 blood space. 



With corn starch injecting mass these spaces will frequently 

 inject for half the width of the gill. With gelatine injecting 

 mass it is quite possible to inject the vessels of the smaller fila- 

 ments as well and get the connection through the interfilamentar 

 junctions. In these injections it frequently happens that a fila- 

 ment will be injected for only a portion of its length and in such 

 cases the whole cavity of the filament is filled as far as the injec- 

 tion extends. This indicates that the apparent partition is not 

 functional as a division between vessels. 



25 



