592 E. W. MAOBRIDE. 



Ophiuroidea and Echinoidea, the coelomic rudiment first 

 divides into right and left halves, and then these sub- 

 sequently divide into anterior and posterior portions. This 

 has been doubted in the case of Asteroidea. From Agassiz's 

 figures (1) some have drawn the conclusion that in the 

 Bipinuaria larva the right and left coelomic sacs originate as 

 independent evaginations of the gut. But an inspection of 

 these figures shows that Agassiz did not observe enough 

 stages to warrant such a conclusion, and Driesch (8), who 

 has used the larva? both of Asterias and Astropecten for 

 experimental embryology in all cases shows a single vesicle 

 as the first rudiment of the coolom. The course of develop- 

 ment in Asterina gibbosa (and apparently in Cribrella 

 oculata also) differs from that of the Bipinnaria larva in 

 that the anterior portion of the coelom never divides into 

 right and left halves; whereas in the Bipinnaria it first 

 divides into right and left coelomic sacs, and then these 

 reunite in the prse-oral lobe to form the single anterior 

 coelom. It is obvious that in Asterina and Cribrella the stage 

 when the anterior coelom becomes divided into right and left 

 halves is missed out owing to the shortening of development. 

 If we regard the free-swimming larva as representing a 

 bilaterally symmetrical ancestor we may then with confidence 

 consider that the development of the coelom, according to the 

 scheme which I have now shown to obtain in Asteroidea, 

 Ophiuroidea and Echinoidea, gives us the best idea of the 

 condition of the coelom in this ancestor. We must credit this 

 ancestor with a long pra3-oral lobe, which is not found in 

 the Ophiurid because in Crinoidea and Asteroidea this lobe 

 is used as a fixing organ or stalk, and a fixed stage in the 

 development supplies us with the best idea of a transitional 

 condition between a bilaterally symmetrical animal swimming 

 freely and a radially symmetrical creeping animal. At the apex 

 of this pra3-oral lobe there was a brain in the form of a neuro- 

 epithelial plate ; such an organ I have not been able to find 

 in the Ophiopluteus, but it exists in the Crinoid larva and in 

 the larva of Echinus esculentus and as a thickened plate 



