294 



L WER IN VER TEBRA TES. 



Two lai'ge eyes are also formed, which are remarkable in being behind the velum. 

 The details of the closure of the blastopore, the formation of the pedal nerves (of too 

 technical a character for recital here), 

 the bilateral symmetry and the segmen- 

 tation of the body, all point to the fact 

 that the chitons branched off from the 

 gastrojiodous stem at an early date. 



The chitons are mostly 



littoral forms living in the 



shallow waters of the 



ocean. Over three hun- 

 FiG.33o.-Trof/!,v dred species are known ; but until the manuscripts of the late Dr. P. P. 

 redchito™'""^' Carpenter are edited and published, we shall have no adequate review 



of the group. A large number of genera have been made, but with 

 these we need not concei'n ourselves. 



Fig. 329. Development o£ Chiton. 



Super-Order II. — ANISOPLEURA. 



In this, by far the largest division of the Gasteropoda, the symmetry so marked in 

 the preceding group is greatly obscured. The head and foot, indeed, retain the prim- 

 itive bilaterality ; but here the resemblance usually ends. The cause of this lack of sym- 

 metry in other parts of the organism is to be explained on mechanical grounds. On 

 the back there is usually developed a lai-ge shell, which, with its included viscera, ac- 

 quires a very great proportional weight. This shell naturally falls over to one side, 

 and by thus doing twists the various organs so that the primitively median anus occu- 

 pies a position at the anterior portion of the body, usually upon the right side, or may 

 even be placed in the median line above the head. Not only is the alimentary tract 

 affected by this torsion, but the openings of the kidneys, the gills, and other oi-gans are 

 transposed, so that the gill, for in- a E c 



stance, of the normal right side is in 

 reality borne upon the left. Part of 

 the nervous system may or may not 

 share in this twisting, accordingly as 

 the visceral loop is above or below 

 the anus. The effect of this twisting 

 is to coil the nerves in the shape of 

 the figure 8, and an illustration of the 

 stages of the process may be seen in 

 the adjacent diagrams copied from 

 Lankester who was first to jioint out 

 the systematic importance of these 

 facts. Coincident with this torsion 

 frequently occurs an atrophy of parts, 

 and, from the fact that the twisting usually occurs in one way, it is the gills, kidneys, 

 etc., of the left side which usually suffer or even entirely dis.appear. 



The twisted or straight character of the visceral nervous loop gives a founda- 

 tion for a division of the Anisopleura into two groups, to which the names Streptoneura 

 and Eulhyneura have been applied. To the former belong the great majority of the 



Fig. .331. — Diagram showing the torsion of the body when the 

 viscer.ll commissure passes above the intestine; A, nor- 

 mal condition; B, quarter rotation; C, complete half 

 rotation; a, .luus; (, left, r, right renal organ. 



