Metazoa A mphioxus. 253 



phioxus lanceolatus is from i^ to 2 inches in length, and 

 when living is delicate, translucent, and fish-like. Its body 

 is compressed from side to side and pointed at both ends. 

 No limbs are present. A series of small and delicate fila- 

 ments may be seen protruding from the anterior end. A 

 narrow ridge or fold of skin, known as the dorial fin, runs 

 along the back. At the tail end this ridge becomes broader 

 and more distinct, and is there termed the caudal fin. 

 On the ventral surface the caudal fin is continued forwards 

 for a certain distance ; finally, however, in the anterior two- 

 thirds of the ventral surface it is replaced by two ridges 

 separated by a groove. 



The mouth is an oval slit on the ventral surface of the 

 body, near the anterior end, from the margin of which spring 

 the filaments already referred to. Two other apertures are 

 to be found on the ventral surface : the anus, situated a 

 little to the left of the middle line, just in front of the caudal 

 fin, and an aperture known as the atrial pore, situated at 

 the point where the ventral continuation of the caudal fin 

 ends. The sides of the body are obliquely marked, the 

 lines indicating the outlines of muscle bands or myotoines, 

 and running dorsaily and ventrally from the median line 

 backwards. It is important to notice this point, as it exhibits 

 to us one of the points of evidence of the origin of the Chor- 

 data from segmented worm-like ancestors. 



Ihe alimentary and respiratory systems. As in the 

 worm, the alimentary canal is straight and even simpler than 

 it was in that type. The mouth opens into a buccal cavity 

 lined by ciliated epithelium. The buccal cavity is separated 

 posteriorly from the pharynx by a circular fold of membrane 

 or diaphragm. This diaphragm is pierced by an aperture 

 fringed by numerous ciliated lobes. The buccal cavity 

 opens into the pharynx, or anterior portion of the alimentary 

 canal, and it is here that we meet with a great point of dif- 

 ference between the worm and ^nftj$tp$us. Unlike that of 

 the worm, the pharynx ijS^fmfely anm^ntary in function 



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