ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 277 



In the chamber of this floating box we placed numerous wide-mouthed glass jars, 

 containing prism-larvae, derived from fertilizations made in the laboratory. The 

 mouths of these jars were covered with a coarse-meshed bolting silk, which had been 

 put through a special tanning process, to prevent it from rotting when in sea- water. 

 The mesh of this net was large enough to allow diatoms and other small algse to pass 

 through, while it prevented the escape of the larvae. The chambers of the raft were 

 so constructed that the jars were freely washed by the changing tidal water, and at 

 the same time were sheltered from direct sunlight, and were completely submerged 

 beneath the svu'face of the sea. 



In 1911 cultures of plutei were kept in the raft, but, although they metamorphosed 

 there, their rate of growth was not so rapid as that of similar cultures in the 

 laboratory. The larvae were examined from time to time, and their stomachs always 

 appeared to be empty. Doubtless the comparatively slow rate of growth was due to 

 the poor food supply. The waters of the land-sheltered bay, in which the floating 

 box was anchored, possessed a flora much poorer in diatoms than those of the open 

 sea, which is the natural habitat of the plutei. Moreover, the diatoms will not 

 readily pass through the silk netting into the jars. In the laboratory, while many of 

 the conditions, such as confinement within jars, temperature, etc., are highly 

 abnormal, the plutei can at least be supplied with as much food as they can digest, 

 which could evidently not be supplied in the raft. 



During the summer of 1912 a diflPerent plan was adopted. Some young ^c7«'m, 

 which had just metamorphosed in the laboratory, were placed in glass jars and 

 confined there by a much coarser-meshed silk than was necessary for the plutei. 

 These young sea-urchins were left out in the raft for one month. Measurements 

 showed that they had remained absolutely stationary as regards growth during that 

 time, while individuals from the same culture kept in the laboratory had increased 

 rapidly in size. No algae or other food-stufls seemed to grow in these jars kept in 

 the open sea ; whereas, as explained above, the Echini raised in the laboratory are 

 supplied with abundant food material. 



6. Characters of the Pure-Bred Plutei. 



The rates of development of both pure-bred and hybrid Echinoid larvae vary 

 within wide limits, and this variation is dependent on a number of circumstances. 

 The state of the germ cells at the time of fertilization, the temperature, and the food 

 supply, are all important factors which influence the rate of growth. Excluding the 

 unhealthy larvae which lag behind, the rates of growth of the individuals in a given 

 culture jar are not very difterent from one another, but different cultures made from 

 the same fertilization may develop at very different rates. The plutei in most 

 cultures reach the stage of metamorphosis from one and a half to two months after 

 fertihzation, but some may not metamorphose for five months. When the develop- 

 ment is as slow as this the Echini are usually weakly. Suflicient has been said, 



