568 E. W. MACBRTDE. 



velopini^ larva) obtaiued plenty of food. Returning to com- 

 plete my work on Echinus esculent us in 1899, I again 

 tried to rear the larva3 of Ophiothrix f ragilis, but although 

 I obtained ripe adults and fertilised the eggs I was unable to 

 keep the larvte living for longer than five days. Towards the 

 end of my stay the larvte became very abundant in the Plankton, 

 and from July 14th to 19th, 1899, it might be fairly said to 

 swarm with them. As I observed that the Echiuoderm larvas 

 were in a somewhat pathological condition when the bottles 

 containing the Plankton were brought into the laboratory, I 

 went out in the steamer belonging to the station and pre- 

 served the Plankton immediately on its being brought to the 

 surface. For this purpose I used 1 per cent, osmic acid 

 followed by Miiller's fluid — a method which has the advantage 

 of giving an excellent preservation of the tissues, but it 

 labours under two disadvantages : it makes the specimens 

 brittle and it completely dissolves the calcareous matter, and 

 hence no attention is paid in this paper to the development of 

 the adult skeleton. The Plankton contained in all four varieties 

 of Ophiurid larva3, but the larva} of Ophiothrix fragilis 

 were in an enormous majority, and in all stages of develop- 

 ment up to the completed metamorphosis. They were easily 

 distinguishable by the greatly developed postero-lateral arms, 

 which in this species are extraordinarily long. In this way 

 a large amount of material was obtained, but I was unable to 

 commence work on it till 1903, when I had completed the 

 work on Echinus esculent us. In 1905 I paid another 

 visit to Plymouth in order to fill the gap in the series of 

 stages which I had obtained from the Phnikton. It happened 

 that the season was rather later, so far as the breeding habits 

 of this species were concerned, than it had been in 1898 and 

 1899, and though I examined hundreds of adults I only 

 succeeded in obtaining two females Avhich were capable of 

 fertilisation. Of these the first gave larvae which pursued a 

 development similar to that undergone by the larvae which I 

 obtained by artificial fertilisation in 1899, and which I believed 

 to be the normal course of events, but I was unable to keej) 



