THE LAMPREY EELS AND OTHER CYCLOSTOMES 269 



attach itself to solid objects. On and around the tongue are 

 horny teeth used for rasping the food. Lampreys are parasites, 

 living on the blood of other animals, principally fish. They 

 make a hole in the victim's body and suck out the blood. 



Respiration. Lampreys breathe 

 in the water by means of gills. 

 There are seven pairs of circular 

 respiratory openings just back of 

 the eyes (Fig. 138, A) ; each open- 

 ing is the entrance to a sac in which 

 the gills are situated. Water enters 

 and is forced out through the same 

 openings, and, as in the crayfish, 

 the blood in the gill filaments ex- 

 changes its load of carbon dioxide 

 for a fresh supply of oxygen that 

 is mixed with the water. 



Sensations. Lampreys are able 

 to see, hear, smell, taste, and feel, 

 but none of their senses is very 

 highly developed. The eyes are poor; the ears have only two 

 semicircular canals instead of the usual three; there is a single 

 nostril situated on top of the head between the eyes; and a few 

 taste cells inside the pharynx. 



Internal Organs. The internal organs are likewise primi- 

 tive. The skeleton is entirely of cartilage; there is no distinct 

 stomach; the heart has only one auricle and one ventricle; and 

 the brain is very simple, resembling that of the embryo of higher 

 vertebrates. 



Development. The development of the lamprey is very 

 interesting. The eggs produce larvae which differ in many 

 respects from the adult, and were at one time considered a dis- 

 tinct species of animal. The larvse lie buried in mud and sand, 

 and food particles are drawn into the mouth by means of a cur- 

 rent of water produced by cilia. In the winter of the third or 



FIG. 154. The mouth of the 

 lamprey eel. (From Forbes.) 



