294 



LOWER INVERTEBRATES. 



Two large eyes are also formed, which are remarkable in being behind the velum. 



The details of the closure of the blastopore, the fonnation 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 



gastropodous stem at an early date. 



The chitons are mostly 

 littoral forms living in the 

 shallow waters of the 

 ocean. Over three hun- 

 dred species are known ; but until the manuscripts of the late Dr. P. P. 

 Carpenter are edited and published, we shall have no adequate review 

 of the gi"oup. A large number of genera have been made, but with 



these we need not concern ourselves. 



Fig. 329. Development of Chiton. 



FlG.330.— Tracftj/- 

 dermon ruberj 

 red cUton. 



Supee-Oeder II. — ANISOPLEURA. 



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

 the preceding group is gi'eatly obscured. The head and foot, indeed, retain the prim- 

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

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

 the back there is usually developed a large 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 organs are 

 transposed, so that the gill, for in- ^ B 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 point 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 disappear. 



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 Euthyneura have been applied. To the former belong the great majority of the 



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

 visceral commissure passes above the intestine; A, nor- 

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

 rotation; a, anus; I, left, r, right renal organ. 



