502 



CHORD ATA. 



inwards, converting the plate into a tube with nervous walls and 

 a central canal. From this, as will readily be seen, it happens, 

 when the blastopore remains open behind (fig. 547, ne), that a 

 temporary communication, the neurenteric canal, exists between 

 the neural and alimentary canals. 



On the other hand the chordates share with the annelids and 

 arthropods a segmentation of the body which, however, is internal 

 and only exceptionally is visible from the surface. 



The Chordates include the Leptocardii, the Tunicata, doubt- 

 fully a group of Enteropneusta, and the Vertebrata. 



SUB PHYLUM I. LEPTOCARDII (CEPHALOCHORDIA, ACRANIA). 



Until recently but a single genus (Amphioxus) was recognized 

 as belonging to this group, and this form, known for over a hun- 

 dred years, was at first described as a mollusc (Limax lanceolatus). 

 Its chordate nature was first recognized by Johannes Muller, 

 while the embryological researches of Kowalewsky showed its close 

 relations to the Tunicata. 



In structure it is comparatively simple. The fish-like body, 

 pointed at either end (whence Amphioxus), lacks paired appendages, 

 but has a median fold or fin best developed at the caudal end. The 

 epithelium covering the body is but a single cell in thickness and 



FIG. 541. Amphioxus lanceolatus. (Diagram after Boveri.) a, anus ; nu, eye ; />, peri- 

 branchial space; c, notochord ; 0, gonads ; /, liver; m," muscles ; ?i, nephridia ; o, 

 mouth ; p, atrial opening ; r, spinal cord ; sp, gill slits. 



allows the underlying muscle segments to show through. It differs 

 from the fishes in lack of skull (Acrania), vertebrae, brain, heart, 

 and kidneys, although the rudiments of brain and excretory organs 

 are present. Connective tissue is almost entirely absent, the body 

 consisting of much-folded epithelia separated by thin gelatinous 

 layers. 



An axial skeleton is present in the notochord, which extends the 

 whole length of the body (fig. 541, c). Above it lies the spinal 

 cord, with a central canal, which expands in front into a rudi- 

 mentary cerebral vesicle. A pigment spot in this brain is the 



