234 E. W. MACBRIDE. 



as a sac separate from the anterior coelom ; he states that the 

 mesentery hetween the right and left coelomic lobes is absorbed 

 ventrally. We have seen that only the posterior parts of the 

 right and left coelomic lobes are employed in the formation of 

 the right and left posterior coeloms respectively; the anterior 

 parts of these lobes are continuous with the anterior coilom, 

 and the longitudinal mesentery between them breaks down, as 

 Ludwig observed. Hence we see that the hinder part of the 

 anterior coelom in Asterina is at first a double structure; in 

 the Bipinnaria larva the anterior ca3lom is at first double 

 throughout its whole extent. 



At the dorsal anterior angle of the left coelom (fig. 37) an 

 invagination of its wall takes place, giving rise to a thick- 

 walled vesicle {o7\ c), which communicates by a narrow slit 

 with the coelom. This structure has been strangely misunder- 

 stood. Ludwig saw it, but not its origin, and supposed it to 

 arise as a *' schizocoele," and regarded it as the rudiment of 

 tlie oral blood-ring. In my preliminary account I recognised 

 its true nature, but supposed that its upper end was the rudi- 

 ment of the so-called heart,^ with which, as a matter of fact, 

 it has nothing to do. It is the rudiment of the oral coelom, a 

 space closely surrounding the adult oesophagus, the relations of 

 which we shall study later. 



Histology of the Larva. 

 The structure of the body-wall of the larva is shown in 

 PI. XX, fig. 138, and PI. XXI, fig. 144. In the first we see 

 that the peritoneum of the left posterior coelom consists of 



' It will be observed that Bury, in his last paper (' Q. J. M. S.,' September, 

 1895), makes the same mistalie. This work appeared after tlie present paper 

 had been sent in for publication, and is therefore not referred to further here. 

 The best answer to Burj's criticisms on my observations as recorded in the 

 preliminary account (15) is the publication of full details in the present paper. 

 Bury's observations contain much interesting matter, but also in my opinion 

 many mistakes, which are due to the fact that the stages which he obtained in 

 the development of most of the larva; he studied, did not form a series without 

 gaps ; the orientation which he adopted seems to me also not that which yields 

 the best results. 



