252 



Elementary Biology. 



FIG. 125. Amphioxiis lancco- 

 latns. (Milnes Marshall.) 



A, buccal cavity; B, buccal ten- 

 tacles ; c, pharynx ; D, liver ; 

 E, stomach ; o, intestine ; H, 

 anus ; i, atrial pore ; K, noto- 

 chord ; L, spinal cord ; M, eye ; 

 N, septa ; o, dorsal fin ; P, ven- 

 tral fin ; R, transverse muscles 

 in floor of atrial cavity. 



into the form of a nervous cord 

 lying along the ventral surface of 

 the body provided \vith a large 

 ganglionic enlargement anteriorly 

 and a series of smaller ganglia 

 posteriorly on the cord, one in 

 each somite. In the early 

 Chordata these lateral cords 

 seem to have become approxi- 

 mated on the dorsal side of the 

 body, and formed what are 

 known among the Vertebrata as 

 the spinal cord. Further, the 

 spinal cord became supported 

 and strengthened by the forma- 

 tion underneath it of a bar 

 known as the notochord. 



It is indeed fortunate that we 

 possess in Amphioxus lanceolatus 

 a minute fish- like animal, found 

 abundantly burrowing in sand in 

 the Mediterranean and other re- 

 gions, a survival of some ancient 

 group of the Chordata, compara- 

 tively speaking not far removed 

 from this primitive type. We 

 shall give an account in the first 

 place of this form, and then point 

 out some of the conclusions 

 which may be drawn from its 

 study with regard to the various 

 and important differences which 

 we notice existing between the 

 vertebrate and the invertebrate 

 types. 



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