1 3 6 



BIOLOGICAL LECTURES. 



The ear of Bdellostoma lies imbedded in the cartilage of the 

 base of the skull and below a thick layer of muscle. The 

 membranous portion is essentially like the ear of Myxine, 

 and the reader is referred to the previous volume of lectures 

 where I have described the Hagfish's ear in considerable detail. 



Bdellostoma lacks all traces of the 

 system of lateral line organs so far 

 as is yet known, and it has no rep- 

 resentatives of the other specialized 

 sense organs of the skin, common 

 to most fishes. 



Isolated sensory cells are the only 

 special sensory structures yet found 

 in this animal's skin. The absence 

 of these surface sense organs, to- 

 gether with the entire lack of 

 appendages, renders Bdellostoma an 

 extremely valuable animal with 

 which to perform ear-function ex- 

 periments; this is due in great part, 

 of course, to the very simple condi- 

 tion of the ear itself. 



Upon cutting through the scale- 

 less and relatively thin skin along 

 the ventral line, the body muscles 

 come to view. Of these, there are 

 two principal layers covering the 

 whole body. When they are cut through along the ventral 

 line, from the mouth to the cloaca, most of the viscera are 

 exposed. In the branchial region, as shown in Fig. 9, we find 

 the heart with its large ventral aorta running forwards, giving 

 off gill-arteries as it goes, in such a manner that either side 

 receives as many branches as there are gills on that side. The 

 gills themselves are compressed sacks, circular in outline, which 

 assume a sub-globular shape when they fill with water. The 

 current of water which we have traced from the nose into the 

 mouth, passes back through the oesophagus, and then out into 

 the gills by as many short tubes leading from the oesophagus 



FIG. 4. A full front view of 

 the face of Bdellostoma. ;/, nasal 

 aperture, i, 2, 3 and 4, the tenta- 

 cles or feelers. / and <>, the inside 

 and outside rows of teeth. ///, the 

 mouth. 7", the tongue, which is 

 here thrust forwards out of the 

 mouth and nearly conceals the 

 opening. The tongue stands here 

 at right angles, in two directions, 

 to its usual position inside of the 

 head. nat. size. 



