232 E. W. MACBRIDE. 



Later stages of this organ are seen in figs. 35 and 36. In 

 fig. 35 it is a conspicuous solid bud; in fig. 36 it has acquired 

 a lumen, and is connected with the anterior ccelom by a string 

 of cells, which soon atrophies, and it is then left as an isolated 

 vesicle in the midst of the mesenchyme. Bury (2), indeed, has 

 seen it in this stage, and called it " a mesenchymatous vesicle;" 

 and Field (5) has described what I believe to be an homologous 

 structure in the larva of Asterias. The right hydrocoele persists 

 in the adult as a closed sac just under the madreporite, and 

 has been seen here by Cuenot (3), and Leipoldt (9) has described 

 a similar sac in Echinids. It may seem rather a rash assump- 

 tion to regard this orfjan as the fellow of the water-vascular 

 system, but a complete proof that this is really its nature will 

 be given when abnormal larvse are described. 



Stage D, the summit of the development of the larva, is 

 reached on the seventh day, according to Ludwig (PI. XI, figs. 

 10 and 11). The prseoral lobe and the larval organ have 

 greatly increased in size, the former having acquired a large 

 ventral as well as a dorsal lobe. The internal changes are 

 more striking than the external. The separation of gut from 

 coelom was practically complete in Stage C, the last trace of 

 connection being shown in fig. 36. The right posterior coelom 

 is entirely separated from the anterior coelom, but, strange to 

 say, the septum between the left posterior coelom and the ante- 

 rior coelom has become broken down in two places. This occurs 

 by the two layers of epithelium of which it is composed fusing, 

 and then thinning out to a film. Of these two secondary com- 

 munications between the two sacs, one is situated dorsal to the 

 left hydrocoele (PI. XIII, fig. 42), and the other ventral to it 

 (PI. XII, fig. 41). Figs. 42 and 43 belong to the same series; 

 we see that the dorsal opening is formed before the separation 

 of the right posterior coelom is complete ; the ventral opening 

 is formed at the same time. Not having had the opportunity 

 when I wrote my preliminary account (15) of observing younger 

 larvae than these, I imagined that the segmentation of the 

 coelom of the left side was incomplete ab initio, a mistake 

 which was the more excusable as both the breaches in the 



