THE ELASMOBRANCH FISHES 217 



The first dorsal may be taken as an example of the 

 skeleton of the median fins (fig. 52). Its skeleton is com- 

 posed of twelve backwardly directed basal cartilaginous rods 

 attached by ligament to as many neural spines. These are 

 succeeded by as many somewhat longer and stouter radial 

 rods, and these in turn are succeeded by two rows of polygonal 

 cartilaginous plates which support the actinotrichia. The 

 second dorsal and ventral median fins are formed on 

 much the same plan as the first dorsal, but in the caudal 

 fin the radial and basal cartilages have disappeared, their 

 place being taken by the elongated flattened spines of the 

 neural and haemal arches, which afford sufficient support 

 to the actinotrichia. As fin-structure is of great importance 

 in the classification of fishes, it is essential that the foregoing 

 description should be thoroughly understood by the student 

 of comparative anatomy. 



Such ample directions for the dissection of the dogfish are 

 given in Marshall and Hurst's " Practical Zoology " that there 

 is no need to recapitulate them here, but attention must be 

 directed to those features which are of importance from the 

 point of view of comparative anatomy. 



The ccelom is spacious, but is confined to the trunk. In 

 the caudal region, behind the anus, the whole thickness of 

 the body is made up of the myotomes surrounding the 

 vertebral column. The only spaces are those enclosed by 

 the neural and haemal arches. The former is occupied by 

 the spinal cord, the latter by the caudal artery and vein, and 

 the narrow space surrounding these can scarcely be regarded 

 as ccelom. Nor is there any trace of ccelomic cavities in the 

 head or in the gill-region in the adult animal, but in the 

 embryo dogfish at least five ccelomic pouches are discover- 

 able in front of the auditory region, and an uncertain number 

 in the post-auditory region of the head, and it has been 

 shown that the ventral or splanchnoccelic moieties of some 

 of these cavities extend down into the partitions between the 

 gill-slits. This indicates that the visceral arches belong to 

 the ventral part of the head, and, in the course of specialisation 

 and modification have been shifted backwards, so that the five 

 branchial arches lie beneath the anterior segments of the 

 trunk, and only the jaws and hyoid arch- retain their connec- 



