lO 



CIIORDATE ANATOMY 



are limited to the gill region. As in urochordates, the gills open into a 

 peribranchial cavity. Development involves metamorphosis. 



There are possibly 25 species. 



AMPHIOXUS. The lancelet, Amphioxus, the characteristic genus of 

 the group and the so-called connecting link between vertebrates and 

 invertebrates, interests morphologists because of its resemblance to the 

 hypothetical ancestor of vertebrates. Its resemblance to the larva of 

 cyclostomes is impressive. (Fig. 11) 



The Amphioxus is a lance-shaped animal, not more than two inches 

 long, with a laterally compressed body and a median caudal fin. Its 

 external orifices are an anterior ventrally placed mouth, an anus to the 

 left of the caudal fin, and an atriopore somewhat behind the middle of the 



CIRRI 

 ORAL HOOO! 



Fig. II. — Amphioxus, in ventral and side views. Metamerism, lacking in uro- 

 chordates, and scarcely evident in hemichordates, is strikingly shown by Amphioxus. 

 Whether this metamerism is inherited from annelid-like ancestors or is a convergent 

 trait independently acquired, is a moot question in morphology. (Redrawn after 

 Kirkaldy.) 



body. The atrial chamber which surrounds the elongated pharynx is 

 formed by the union of paired lateral folds which meet in the mid-ventral 

 line of the body. Such a structure seems to be an adaptation to the 

 sand-burrowing habit of the adult animal. The atrial cavity ends blindly 

 in front, and opens externally by the atriopore just behind the pharynx. 

 In the region of the pharynx, a pair of ventro-lateral metapleural folds 

 extend as far back as the atriopore. 



The body is covered by a thin external cuticula secreted by the simple 

 epithelial epidermis. Beneath the skin and visible through it are sixty 

 pairs of myotomes which alternate with one another along the two sides 

 of the body. As in the vertebrates generally, these myotomes are greatly 

 thickened along the dorsal side of the body. Each myotome is V-shaped 

 with the apex of the V pointed forward. 



The mouth, surrounded by a circle of tentacles, leads directly into 

 the elongated pharynx, the walls of which are perforated by numerous 



