BDELLOSTOMA DOMBEYI, LAC. 



141 



especially with respect to the branchial 

 is shown in the cut. It is through the 

 that the blood is carried to the walls 

 of the gill sacks, by means of the 

 individual branches a, one of which 

 leaves the main trunk, so long as it 

 remains unpaired, for each gill. 



Owing to the possibility of its play- 

 ing an important part in the develop- 

 ment of the variability of the gills, it 

 is desirable to examine the nature of 

 the so-called tongue and its muscles a 

 little more in detail than we have yet 

 done. In Figs. 9 and 5 these structures 

 are shown as they appear upon slitting 

 open the body muscles. The tongue 

 forms a somewhat heart-shaped plate, 

 when spread out flat as in Fig. 4, and 

 is composed of two .symmetrical lateral 

 pieces. These halves each bear two 

 rows of teeth, and during life, while 

 in the mouth, they are inclined at an 

 angle to each other, thus forming a 

 trough, whose shape is maintained by 

 the sides of the mouth. When, during 

 life, the tongue is thrust out of the 

 body, it is flattened out upon the 

 anterior edge of the solid floor of the 

 mouth cavity, as shown in Fig. 5, 

 when it is in the best possible position 

 for the use to which it is put, vie, the 

 rasping away of the flesh of the fish to 

 which Bdellostoma has fastened itself; 

 or, if it is a small bait, the rapid and 

 certain forcing of the bait into the 

 mouth cavity. The mechanism which 

 effects this end is quite complicated, 

 and I need not enter into a complete 



arteries, whose position 

 large median vessel A, 



^\-^ 



to 



© 15 



Fig. 9. — A ventral view 

 of a dissection of the gills 

 of a Bdellostoma dombeyi, 

 with 12-13 gills, to show their 

 relations to each other, to 

 their blood supply, and to 

 tlie surface of the body. J/, 

 the club-shaped tongue mus- 

 cle. L, its tendon, g. s., 

 l:)ranchial sack, /-/j, the ex- 

 ternal branchial pores or gill 

 holes. T>:, the pores of the 

 thread glands. A, the ven- 

 tral aorta. <?, the branchial 

 arteries. I), the ductus oeso- 

 phago-cutaneus. /»: A, the 

 internal branchial tube. //, 

 the heart. 



