THE DOGFISH 3 



Note the minute openings of the mucous canals distributed over 

 the head, which appear as fine points from which mucus can be 

 squeezed ; they are sense organs. 



On the under side is the crescentic mouth, armed with several 

 rows of teeth. In the smooth dogfish the teeth are flat and plate- 

 like; in the spiny dogfish they are sharp. The nostrils are a pair 

 of irregular openings in front of the mouth. In the skate each nos- 

 tril is connected with the mouth by a groove. Probe the nostrils 

 and determine that they are blind sacs and have no communication 

 with the mouth cavity. Distributed over the ventral surface are 

 the ventral mucous canals. 



The Fins, or Appendages. Two kinds of fins are present, paired 

 fins and median fins. The median fins are the more primitive struc- 

 tures, and are the only ones present in the lowest fishes ; they are 

 simply dorsal and ventral flattened projections of the body. In 

 the smooth dogfish there are two dorsal fins, one ventral or anal 

 fin, and a heterocercal caudal fin, the latter being both dorsal and 

 ventral. In the spiny dogfish each dorsal fin is provided with a 

 sharp spine, and no anal fin is present. 



The paired fins are horizontal, flattened projections extending 

 from the lateroventral surface of the trunk ; they are homologous 

 to the extremities of the higher vertebrates. Two pairs are present, 

 —an anterior pair, the pectoral fins, and a posterior pair, the ven- 

 tral fins; each is supported by a cartilaginous arch within the 

 body wall, called the pectoral girdle and the pelvic girdle respec- 

 tively. They are easily felt through the skin. In the male the ven- 

 tral fins are much longer than in the female ; the inner border of 

 each is more or less separated from the remainder and forms a 

 long, rodlike extension backward. This is called the clasper, and 

 is of use when the animals mate ; observe its structure. 



The ventral fins lie close together and inclose a large median 

 opening, the anus ; this is the outlet of the cloaca, a wide, shallow 

 space which receives the discharges of the digestive and urogenital 

 organs. A pair of small passages, called abdominal pores, one of 

 which is on each side of the cloaca just within its lateral border, 

 form a means of communication between the abdominal cavity 

 and the outside; their function is unknown. Probe them. Just 



