III. 8 



CHEMICAL COMPOSITION 



137 



When potassium salts are added to the sea-water for 

 example, a 7 % solution in sea-water of a 3-7 % solution of KC1 

 in tap- water the egg gives rise to a Pluteus in which, though 

 the gut is, as normally, tripartite, the skeleton is rudimentary 

 and the arms suppressed (Fig. 71). Herbst suggests that the 

 suppression of the arms is due to the absence of a stimulus 

 normally exerted by the skeletal spicules. These abnormal forms 

 may fuse together to form double monsters. 



Such ' potassium ' larvae are developed in sodium salts, but 

 lithium has a more pronounced effect (Figs. 72, 73). In this case 



a 



FIG. 71. Potassium larvae of Echinoids. . Potassium larva of Sphaer- 

 echinus (1860 c.c. sea-water -f 140 c.c. 3-7% KNO S ). There is no skeleton. 

 The gut is tripartite, and the mouth surrounded by the ciliated ring. 

 b, c. Potassium larvae of Echinus (20% of 3% KC1). Note the button- 

 shaped apical tuft of cilia, and, in c, the secondarily evaginated archen- 

 teron. (After Herbst, 1893.) 



the blastula becomes constricted into two portions, a thin-walled 

 gastrula wall provided with long cilia, and a thick"- walled archen- 

 teron, which may be muscular and mobile, and is thickly covered 

 with short cilia. The archenteron has, in fact, failed to in- 

 vaginate, and the larva is an ' Exogastrula '. Occasionally there is 

 an attempt at invagination at the end of the archenteric portion, 

 and, after temporary exposure, the invaginated part may be 

 divided into three, and a mouth formed. All the parts of the 

 gut, however, remain in the same straight line. A middle 

 section may be formed by further constriction of the archen- 

 teron (Echinus) or of the gastrula wall (Sphaerechinus), Double 



