1^6 



BIOL OGICA L LECTi l^ES. 



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 ]\Iyxine, 

 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 

 Fig. 4. — A full front view of extremciv valuable animal with 

 theface of Bdellostoma. //.nasal ^^,|^i^|-^ ^^ perform ear-function ex- 

 aperture. I, 2, 3 and 4, the tenta- . ..... ^ ^ 



, . , 1 ^u • -^ permients; this is cue m great part, 



cles or feelers, i and 0, the mside ^ ' t> r ' 



and outside rows of teeth, w, the of coursc, to the very simple condi- 

 mouth. T, the tongue, which is tion of the ear itself. 



here thrust forwards out of the tx ,.• ,1 i ti i^ 



, , Upon cuttmg through the scale- 



mouth and nearly conceals the ^ <=> ^ 



opening. The tongue stands here Icss and relatively thin skin along 

 at right angles, in two direction.s, the ventral line, the body muscles 

 to its usual position inside of the ^^^^^ ^^ ^,j^^^, Qf ^^yn^^, there are 



hgQQ n3.t S1Z6 



two ]3rincipal layers covering the 

 whole body. When they arc cut through along the ventral 

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

 exposed. In the branchial region, as shown in b'ig. 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 



