THE LANCELET AND THE PRIMITIVE VERTEBRATE. 453 



coalesced in the middle line of the ventral side (JR) . The respiratory water 

 now passes from the intestinal cavity (D) into the gill-cavity (A). In all, 

 the letters indicate the same parts : N, medullary tnbe ; Ch, notochord ; 

 M, side-mnscles ; Lh, body-cavity ; Q, portion of the body-cavity in which 

 the sexual organs afterwards form ; D, intestinal cavity lined by the intes- 

 tinal-glandular layer (a); A, gill-cavity ; -ZT, gill-openings ; b=E, outer skin, 

 or epidermis ; 1 , the same as the inner epithelium of the gill-cavity; 

 the same as the outer epithelium of the gill-cavity. 



which was taken in at the mouth, passes out, no longer 

 directly through the gill-openings, but through the gill-pore 

 (p. branchialis). That portion of the intestinal canal which 

 is situated behind the gill-body becomes the stomach- 

 intestine, and forms on the right side a single purse-like 

 protrusion, which becomes a blind liver-sac. This digestive 

 portion of the intestinal canal is enclosed in the narrow 

 body-cavity. 



In an early stage of individual development, the struc- 

 ture of the body of the Amphioxus larva still corresponds 

 essentially with our ideal "Primitive Vertebrate." The 

 body afterwards, however, undergoes various modifications, 

 especially in the anterior portion. These modifications are 

 uninteresting to us at present, because they depend on 

 special conditions of Adaptation, nor have they anything to 

 do with the hereditary vertebrate type. Of the remaining 

 portions of the body of the Amphioxus, we need only 

 remark that the germ-glands, or internal sexual organs, do 

 not develop till later, and, as it appears, directly from the 

 inner cell-coat of the body-cavity, from the ccelom- 

 epithelium. Although no extension of the body-cavity 

 is afterwards discernible in the side walls of the gill-cavity, 

 in the gill-roofs (Fig. 152), yet such an extension does at 

 first exist (Fig. 159, 160, Lh}. In the lowest part of this 



