138 



EXTERNAL FACTORS 



III. 8 



monsters sometimes arise by fusion of these larvae by their 

 archentera. 



A skeleton is not usually developed ; if present it is abnormal 

 in position, the spicules being placed near the animal pole and 



FIG. 72. Lithium larvae of Sphaerechinus granularis. a. Larva partially 

 constricted into gastrula wall and archenteric portions, the former with 

 long, the latter with short cilia (980 c.c. sea-water + 20 c.c. 3-7% LiCl). 

 b. Similar larva to the last, but a neck or connecting piece has been 

 formed from the ectodermal portion, c, d. Progressive diminution of the 

 ectodermal gastrula wall portion with increase in the quantity of Li. 



the arms of the Pluteus formed under their influence near the 

 mouth instead of by the side of the anus, in the number of the 

 spicules, and consequently the number of arms (three, four, or 

 five, instead of two), and in the number of their radii (four or 

 five, instead of three). 



