A MANUAL OF ZOOLOGY 43 



culum). Can you distinguish the divisions,' head, body, 

 and tail? How? "What are cervical vertebrae? Would 

 you expect to find them in a fish? Do ydu find a lateral 

 line? Note the position of the mouth. Open it and find the 

 teeth on the upper jaw: They are on the premaxillary 

 bone. How is it fastened to the skull bones? The paddle- 

 shaped bone back of it is the maxillary. - Those consti- 

 tuting the sides of the lower jaw are the dentary bones. 

 Do they bear teeth? In the front of the roof of the mouth 

 is situated the patch of vomerine teeth, so called from 

 the bone upon which they are placed. Extending back 

 from these teeth on each side of the roof of the mouth 

 are two rows of teeth situated on the palatine bones. Are 

 the teeth placed in regular rows? Look at them with the 

 lens. What is their shape, their slant, and their function? 

 Back of the premaxillary bone note a pocket-like flap or 

 valve with the opening turned backward. What is its 

 use? Is there a tongue? 



Examine the position and general appearance of the 

 eyes. From, a superficial examination of the eyes in how 

 many respects do they differ from the eyes of the dog-fish? 

 Do you think that the nictitating membrane in the dog-fish 

 -is an organ in the process of evolution or of degeneration? 

 Why? Press on the roof of the mouth and note effect on 

 the eyes. Was this effect possible In the dog-fish? An- 

 teribr to the eye is situated a thin triangular bone, the 

 ante-orbital bone. Note a pair of openings in front of each 

 eye, the nostrils. Do they open into the mouth? Do they 

 communicate with each other? Are there any valves for 

 closing themi? Compare their situation, with reference 

 to the mouth, to those of the dog-fish. Can you find a 

 spiracle? How many external gill openings are there? 

 Find the operculum or gill covering and note that it is 

 composed of over-lapping parts the posterior of which -is 

 called the opercle. Over-lapping the opercle in front is 

 the pre-opercle. This generally has a serrated hinder edgfe. 

 The inter-opercle lies within its dorsal edge under the oper- 

 cle and the pre-opercle and over the sub-opercle, which 

 is ventral. Considering the fact that there are a number 

 of gill-clefts in the embryo of this fish, what do the parts 

 of the operculum mean? Is the arrangement a better pro- 



