CLASS ACRANIA. 87 



warmer seas, though it not infrequently swims about 

 with a wriggling motion. On examining the structure 

 of the animal, we find that its supporting skeleton con- 

 sists chiefly of a slender notochord, running from head 

 to tail. Above this is the nervous system, slightly 

 enlarged anteriorly, where it bears a single eye-spot. 

 Below, and in front, is a large mouth, surrounded by 

 tentacles, which leads into a chamber, the pharynx (far'- 

 ingks), from the posterior end of which extends the 



FIG. 149. 



s 



Branch ios' to ma Ian ceo la' turn. Amphioxus, or Lancelot, enlarged. E, Eye. JV, Notochord. 

 B, Branchial slits. L, Liver. 6', Nervous system. The arrows indicate the entrance and 

 exit of water. 



oesophagus, which connects with the stomach. Water 

 taken into the pharynx finds its exit through numerous 

 lateral slits, the separations between which are richly 

 supplied with blood-vessels, coming from a ventrally 

 placed pulsating organ. In these supporting arches, the 

 blood is purified by coming so near the water that its 

 impure gases escape, and the free oxygen (ox'ljen) in 

 the water is taken in. These arches are then used for 

 breathing, and we can speak of them as the gill, or 

 branchial, arches. The outer openings of the slits are 

 covered by a fold of skin which forms a chamber around 

 them, having a single external pore. The specimen 

 which is represented in the figure, is from the Mediter- 

 ranean ; but one species, less studied, is found in Amer- 

 ican waters. Six species in all are enumerated by some 

 naturalists. 



