ECHINOCYAMUS PUSILLUS. 



rapidly in number. The process causes no great trouble, because 

 the larvfe as a rule live near the surface or at least at some di- 

 stance from the bottom. Besides, it is not difficult to adjust the 

 thickness of the siphon so as to effect the least possible suction. 

 After this, the aquarium may be filled up to the height desired 

 with a fresh supply of filtered water fetched from the open sea. 

 6) Lastly, the temperature in the laboratory ought not to be too 

 high, for which reason it is not advisable to place the aquaria in 

 sunny rooms. 



It was not till I had availed myself of all these precautions that 

 I succeeded in inducing larvge of Echinocyamus to complete their deve- 

 lopment from the egg to the young sea-urchins which crawled about in 

 great numbers at the bottom of the aquarium. It is evident that the 

 larvae found these arrangements agree with them. They always retained 

 their vivacity and their power of rising and descending in the water, 

 and acquired the normal quantity of pigment. If they were dispersed 

 by stirring the water, they gathered shortly again into masses resembling- 

 bands or festoons which stretched downwards from the surface, each 

 individual changing its place by moving vertically up or down in the water. 

 Besides, the more advanced larvte even swam freely in a horizontal di- 

 rection. The sense of affinity of these larvte is singular to see, since 

 when they have been dispersed in all directions, they gather again in 

 a short time so as to form shoals. This is not due to the currents in 

 the water, as one is inclined to believe to be the case in the open sea, 

 because if the water in the aquarium for several days has been perfectly 

 still, one will find that the larvge in question change their places and 

 almost always present themselves congregated in shoals. 



If it had been possible to prolong my stay at the sea-side, I am 

 fully convinced that I should have been able to get young sea-urchins 

 several months older which had been hatched in the aquaria, because 

 as soon as they have attained this stage in their development, they 

 display a high degree of vigour and strength. The fact is that they 

 have gained effective defensive weapons against their enemies, such as 

 calcareous plates, feet and spines which are movable in all directions by 

 means of strong muscles. 



The time which passes from the fertilization of the egg till the 

 larva has reached the developmental stage when it becomes able to crawl 

 about the bottom, is about two months. The fertilization took place 

 during the last days of June, and the young sea-urchins had attained 



