LESSON XXIX 



THE DOGFISH 



THE animals studied in the three previous Lessons have 

 served to illustrate three widely different types of organiza- 

 tion. The starfish is radially symmetrical, with an under- 

 lying bilateral symmetry, and no indication of metamerism : 

 the crayfish is bilaterally symmetrical, metamerically seg- 

 mented, and provided with numerous limbs, both trunk and 

 limbs being covered with a hard, jointed armour or exo- 

 skeleton : the mussel is likewise bilaterally symmetrical, 

 covered with a shell formed of paired pieces, and having no 

 indication of metamerism, and no trace of limbs. We have 

 now to consider, in the dogfish, an animal belonging to the 

 great group of Vertebrata, in which the bilaterally symme- 

 trical body is definitely divided into metameres, although 

 there is no indication of the fact externally. There are only 

 two pairs of limbs or paired appendages, and the main sup- 

 porting structures aje a complicated internal system of 

 articulated hard parts, forming the endoskekton or internal 

 skeleton. 



The commonest British dogfishes are the Rough Hound 



