I. LEPTOCARDII 



439 



there being no such nervous ring (Enteropneusta excepted) around the 

 cesophagus, as is common in the invertebrata. This nervous system arises 

 as a medullary plate on the dorsal side of the embryo around the blasto- 

 pore. The edges of this plate are rolled inwards, converting it into a 

 tube with nervous walls and a central canal. From this, as will readily 

 be seen, when the blastopore remains open behind (fig. 500, ne), a tem- 

 porary communication, the netirenieric 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 exception- 

 ally is visible from the surface. 



The Chordates include the Leptocardii, the Tunicata, doubtfully a 

 group of Enteropneusta, and the Vertebrata. 



SxTB Phylum I. Leptocardii (Cephalochordia, Aceania). 



The Leptocardii contains a few very similar forms. One of these, 

 originally described as a mollusc, is comparatively simple in structure. 

 The fish-Ilk e body, pointed at both ends (whence Amphioxus) lacks 

 paired appendages but has a median fold, developed into a fin at the 

 caudal end (fig. 493), and there is a ventral longitudinal fold on either 



Fig. 493. — Amphioxus lanceolatus (diagram after Boveri). a, anus; au, eye; 5, 

 peribranchial space;c, notochord;g, gonads; /, liver; m, muscles; n, nephridia; o, mouth; 

 ■p, atrial opening; r, spinal cord; sp, giU slits. 



side of the peribranchial chamber (fig. 494). The epidermis is but a 

 single cell in thickness, paralleled only in invertebrates, and through it 

 the underlying muscle segments can be seen. Amphioxus differs from the 

 fishes in the lack of skull (Acrania), vertebras, brain, heart, and kidneys, 

 although excretory organs and the rudiments of a brain are present. Con- 

 nective tissue is scanty; the body is largely a much folded epithelium, 

 separated by thin gelatinous layers, into which cells wander, the begin- 

 nings of a mesenchyme. 



The notochord, which extends the length of the body (fig. 493, c) is 

 the axial skeleton. Above it is a tubular spinal cord, expanded in front 



