^j2 THE DOGFISH. PHYLUM CHORDATA 



and at the sides two slit-like eyes. Immediately behind each eye 

 is a small, round opening, the spiracle, while farther back and 

 more towards the ventral side is a row of slits which are the gill 

 slits or gill clefts. The spiracle and the gill clefts open internally 

 into the pharynx. Behind the head the body gradually changes 

 its shape, becoming flattened from side to side instead of from 

 above downwards. The vent or opening of the cloaca lies in a 

 deep longitudinal groove of the belly, just before the middle of 

 the body. Into the same groove there opens at each side one of 

 the abdominal pores, which lead from the body cavity. There 

 are two pairs of fins and four unpaired fins. The fore or pectoral 

 fins, corresponding to the arms of man, are a pair of flat, triangular 

 organs attached by one angle to the sides of the ventral surface 

 not far behind the head. The hinder or pelvic fins are smaller 

 and narrower structures of somewhat the same shape, attached 

 one on each side of the middle line of the belly in front of the 

 vent. In the male, their inner edges are fused and there projects 

 backwards from the under surface of each a rod, grooved along 

 its inner side, known as a clasper. The unpaired fins are median 

 structures in the tail. Two, known as the anterior and posterior 

 dorsal fins, are on the back, one, the ventral fin, is on the under 

 side, and another, the caudal fin, surrounds the end of the tail. 

 This fin has two lobes of unequal size, and the skeleton of the tail 

 is turned upwards and passes into the larger upper lobe. This 

 type of asymmetrical tail is called heterocercal. Acanthias has 

 a long spine, from which it gets its name, in front of each dorsal 

 fin. 



GENERAL INTERNAL FEATURES 



Certain generalisations which we have made in the course of 

 the previous chapters enable us to state in a few words a good deal 

 of information about the anatomy of the dogfish. The dogfish 

 is metazoan, triploblastic, coelomate and bilaterally symmetrical. 

 It is also segmented (p. 192), though the primary segmentation 

 is best seen in the early stages of development and is represented 

 in the adult only by the arrangement of the muscles of the body- 

 wall, the segmentation which is found in the backbone and 

 nervous system arising later. It is chordate (p. 309), and lastly it 

 is a backboned or vertebrate animal. This means that it possesses 

 a jointed internal skeleton, and some other features which we 

 shall discuss later. 



