EXTERNAL FORM. I 5 



differential growth of neighbouring structures is a very curi- 

 ous one, and should be borne in mind. It will have a special 

 significance when we come to consider the development. 



There are no paired muscular fins in Amphioxus, but 

 running along the whole length of the back is a median 

 ridge which is called the dorsal fin. It extends round the 

 front end of the body, where it becomes continuous with 

 the right half of the oral hood. (Cf. Fig. 9.) Posteriorly 

 it becomes enlarged to form the tail expansion or caudal 

 fill, and is continued round the hinder extremity of the 

 body past the anus as far as the atriopore. Along the 

 back, this continuous fin is supported by a series of gelat- 

 inous fin-rays, each of which lies in a chamber of its own. 

 The fin-rays, whose number may exceed 250, do not extend 

 to the extreme anterior and posterior ends of the body. 

 The ventral portion of the fin in the region between atrio- 

 pore and anus is supported by a similar series of fin-rays, 

 but there are two of them placed side by side in each com- 

 partment. In other words, the fin-rays of the ventral fin 

 are paired. 



Amphioxus, like most fishes, is laterally compressed so 

 that a transverse section through the body in front of 

 the atriopore is found to have the form of an equal-sided 

 spherical triangle, the base of which consists of the floor 

 of the atrial chamber. At each of the basal angles of 

 the triangle there is a fold of the integument containing 

 a cavity (Fig. 2). This is the metapleiwal fold^ which 

 stretches on each side of the body from the region of 

 the mouth to slightly beyond the atriopore. (Cf. Fig. i.) 

 The cavity in the folds is the mctapkural lympJi-spacc. 

 The apex of the triangular cross-section is formed by 

 one of the dorsal fin-chambers enclosing a lymph-space 

 into which a fin-ray is projecting. 



