314 



METAZOAN PHYLA 



manner of reproduction, returning to asexual reproduction and metagene- 

 sis, which are found elsewhere only among the lower metazoan phyla. 

 They also show a type of colonial life which is more characteristic of the 

 lower phyla. Another striking phenomenon is the marked degeneration 

 which takes place during metamorphosis. 



339. Cephalochordata. — The type of this subphylum is the amphi- 

 oxus, often called the lancelet (Fig. 207). It is a small marine animal 

 reaching a length of but two or three inches and pointed at both ends. 

 It is found on the sandy beaches of tropical and subtropical regions of 

 the world. 



of r7erve cor-ci' 



Dof^sa/ f/f^ rays 

 Ve/unr 



f^erve cord 



P'^/T7enf 

 spo-/- 



No/ochoro/ 



Oral hooc/ 



Oral ■f-eirf-ac/es 



Enc/o- 



' I Atrium 

 Pharynx 



Fig. 209. — Median longitudinal section of an amphioxus. (From Borradaile, "Manual of 

 Zoology," by the courtesy of Oxford Univemity Press.) 



The mouth of the amphioxus opens on the ventral surface of the body 

 near the anterior end, while the anal opening is on the same surface 

 nearly at the posterior end. A fin runs in the median line of the body 

 from the anterior end backward, around the tail, and forward on the 

 ventral surface, passing to one side of the anus, and ends at the atriopore. 

 The atriopore is an opening at a point about one-third of the length of 

 the body from the posterior end. From this point two lateral, ventral 

 folds, known as metapleural folds (Fig. 208), run forward to the region of 

 the mouth. The atriopore is the posterior opening of an atrial cavity 

 which surrounds the ventral side of the body from this point forward and 

 into which open the pharyngeal slits. Water passes from the mouth to 

 the pharynx, through these slits into the atrial cavity, and thence out 

 through the atriopore. Food is secured in the same manner as in the 

 tunicates, a ventral endostyle functioning exactly as in those forms. 



Running dorsally from one end of the body to the other is the noto- 

 chord, a slender rod tapering at both ends and composed of vacuolated 



