274 SYSTEMATIC ZOOLOGY 



capable of secreting enormous quantities of mucus or slime. In 

 one genus the gill slits open directly to the outside, as in the lam- 

 preys ; in another, Myxine, they open into a pair of longitudinal 

 ducts each of which opens to the outside by a single pore. This 

 Myxine, which is about forty-five centimeters long, while living 

 ordinarily buried vertically in the sand, may bore its way into 

 fishes, particularly the cod, and feed on its flesh, thus being a 

 temporary parasite and the only example of this condition found 

 in the Vertebrata. Myxine is said to be hermaphroditic, but 

 recent investigations make this appear doubtful ; the sperma- 

 tozoa are supposed to mature first and the ova later. 



CLASS II. PISCES 



The Pisces (Lat. piscis, fish) include all the fishes except the 

 Cyclostomata. They are commonly called cold-blooded animals, 

 since the temperature of the body is much the same as that of 

 the surrounding medium. They are all aquatic, and consequently 

 gills are always present as organs of respiration. The general 

 shape of the body varies greatly ; there is no movable neck 

 such as is present in many higher Vertebrata, but the head joins 

 the trunk of the body, and the trunk passes gradually into the 

 tail region without any abrupt reduction in size (Fig. 285). In 

 the anterior portion of the body the vertebras are capable of very 

 little motion on one another ; but in the tail they are very mov- 

 able, and so this becomes the principal organ of locomotion. 



The head bears a pair of well-developed eyes and nostrils, but 

 there are no auditory openings, although internal ears are pres- 

 ent ; these are believed to serve the function of orienting the 

 body rather than of hearing. The mouth may be ventral or at 

 the anterior end of the head, and is usually provided with teeth. 

 Most fish have the body covered with scales, and there are 

 always unpaired fins along the median line ; these fins vary 

 greatly in number, but generally consist of one or more dorsal 

 fins, a caudal fin, and one or more ventral or anal fins posterior 

 to the anus on the ventral side of the body ; they are supported 

 by skeletal structures called fin-rays. 



The tail presents several types. The vertebral column may 

 be straight at its posterior end and surrounded by a symmetrical 



