Echinoderm Larvce with Two Water- Vascular Systems. 337 



We may now describe the progress of the cultures in the two plunger 

 jars which contained larvae \yhich had been subjected to the action of hyper- 

 tonic sea-water. When the larvae had attained the age of about 18 days> 

 many of them showed unmistakeable signs of the formation of the rudiment 

 of a right hydrocoele, such as I had observed in the larvae in my cultures of 

 1914 and 1915. 



But then the development of this rudiment stopped, and it looked as if I 

 were about to obtain a result as indecisive as that which I obtained in 1914 

 and 1915. 



This period was characterised by dull rainy weather which did not 

 favour the growth of the Nitzschia in the jars. However, about a week 

 later, sunshine returned, and by means of a persistent addition of Nitzschia 

 culture to the jars the growth of the diatom was maintained in a healthy 

 condition, and at last on August 9 larvaj showing an indubitable right 

 hydrocoele were found in the jars. On August 10, 13 and 14 a thorough 

 examination of the contents of the jars was made. The larvae were taken 

 out in groups of 50, and each group was inspected under the simple micro- 

 scope. In the beautifully transparent larvae any trace of a right hydrocoele 

 was at once clearly visible. In one of the two jars about 2 per cent, of the 

 larvce possessed a right hydrocoele ; in the other, at least 5 per cent, showed this 

 structure. In the plunger jar which served as control only one solitary speci- 

 men provided with a right hydrocoele was discovered amongst hundreds which 

 were examined. 



At the same time some of the larvae which had remained in the " salted " 

 Breftit jars developed a right hydrocoele. These larvae were the offspring of 

 different crosses from the cross the offspring of which was used to form the 

 cultures in the plunger jars. 



When one puts together the results of the experiments made during the 

 four years, no reasonable doubt can be left in the mind of anyone that 

 exposure to the action of hypertonic water at an early stage of development 

 really does bring about the formation of a right hydrocoele. 



It is probable that by a modification of the methods which I employed, 

 the proportion of larvae with two hydrocoeles could be raised to a much higher 

 figure ; but the difficulty of the experiment lies in this, that to produce a 

 right hydrocoele, or at any rate to produce a fully developed one, two factors 

 are necessary, viz.: (1) the exposure to hypertonic water, and (2) the pro- 

 vision of a superabundance of food. Since the growth of a second hydrocoele 

 carries with it the development of a second set of spines and teeth it is an 

 " expensive " structure and makes large demands on the food supply in the 

 body of the larva. 



