336 E. W. MACBRIDE. 



Some three years ago 1 formed the project of making an ex- 

 haustive study of the development of one of the British species 

 of Echinus on similar lines to my worl< on the development of 

 Asterina gibbosa. After several unsuccessful attempts I 

 succeeded this spring in rearing the larvse of Echinus escu- 

 lent us and Echinus miliar is from the egg to the latest 

 pluteus form^ when the processes (arms) are all fully developed, 

 and the first pedicellarise of the adult have already appeared. In 

 every stage of their development these two larvse are easily dis- 

 tinguishable from one another, and, in view of the interest attach- 

 ing to the question as to how far allied species are distinct at all 

 stages of growth, it seemed worth while to publish an account 

 of these larvse before dealing with the more general questions 

 of Echinoid development. My best thanks are due to Mr. Allen, 

 the Director of the Plymoutli Biological Laboratory, and to 

 his staff, for the assistance and advice they rendered me. To 

 the mechanical arrangement for continuously agitating the 

 water, invented by Mr. Allen, such success as I have hitherto 

 obtained is in large measure due. 



The eggs of Echinus miliaris are smaller and more trans- 



Do 



parent than those of E. esculentus, andas a consequence the 

 stage to which it is possible to rear them without special pre- 

 cautions is much less advanced than is the case with E. escu- 

 lentus. Larvae ofE. miliaris have lived for a month without 

 showing unhealthiness, but also without developing a trace of 

 the oral disc (the first trace of adult structure to appear), or 

 even the full number of larval processes. The stage at which 

 larvse of E. esculentus under similar circumstances cease to 

 progress, is that at which there is an unmistakable rudiment of 

 the oral disc of the adnlt. This curious difference throws light 

 on the extent to which the larvse depend for support on material 

 stored up in the ovum. 



The blastulse of E. esculentus are nearly spherical; those 

 of E. miliaris, on the contrary, distinctly ellipsoidal in form. 

 When the invagination which forms the gut has taken place, 

 it is seen that the oval outline of the latter larvse is due to 

 their possession of an enlarged prseoral lobe. This enlarged 



