388 DR ARTHUR T. MASTERMAN ON THE 



Figs. 61 and 62 show clearly the origin of the right posterior ccelom, which can 

 be seen in fig. 61, and even in fig. 50, as a long process lying inside the right 

 lateral ccelom. 



The vertical series (figs. 53-57) exhibits a slightly later stage, besides showing by 

 its different orientation some special points. Fig. 53 is on the extreme right of the 

 larva, and shows only the right lateral and right posterior cceloms. The next fig. shows 

 the latter opening into the main part of the posterior ccelom, and also a portion of the 

 left posterior ccelom, which, as we already have seen, invades the right side. Dorsally 

 to the right lateral ccelom is the tip of the pore-canal (p.c). In fig. 55 the mesenteron 

 opens into the posterior ccelom, the left dorsal and ventral horns of which are cut 

 throughout their length. The pore-canal here can be traced into the stone-canal (s.c.) 

 of fig. 56. In this figure the mesenteron opens into the anterior ccelom, and from this 

 and the next figure the wide extent of the left lateral ccelom may be gathered. 



This completes the structure of stage D, the two other series mentioned being 

 really at the D/E stage, marking the commencement of changes leading to the adult- 

 structure. 



The larva as thus indicated has a curious mixture of characters. It is specially to 

 be emphasised that, with all its distortions and ccenogenetic features, it does not show 

 the slightest trace of axial symmetry. The only deviation from the true bilateral 

 symmetry is in the nature of a left-handed or sinistral asymmetry ; thus the left lateral 

 ccelom (which obviously will become the hydroccele) is much larger than its fellow, 

 though in stage B they were identical in size and appearance. Again, the left posterior 

 ccelom is larger than its fellow, and extends dorsally as well as ventrally. Lastly, we 

 have seen that the central ccelom has travelled to the right side, and (inferentially) the 

 pore- canal has shifted its aperture from the left to the right. 



General names have been used for these various parts of the ccelom because there 

 are at least two possible interpretations of their true homologies, and the use of these 

 merely descriptive names enables the account of their changes to be kept apart from the 

 interpretation. However, to facilitate further description, it may be stated that the left 

 lateral ccelom becomes the ' hydroccele,' and hence the water-vascular system ; the right 

 lateral ccelom forms the epigastric ccelom (Goto) ; the right and left posterior cceloms 

 become the ' hypogastric ccelom ' (Goto) ; the central ccelom forms the cardiac vesicle 

 of the adult, whilst the pre-oral ccelom remains, in part, as the axial sinus. 



Part III. Post-Larval Period. 



The post-larval period consists of two intermediate stages (E and F), in which the 

 larval entity gradually gives place to that of the young starfish. The external character 

 of one stage of the young Cribrella, i.e., stage G, is here added, but the fourth or 

 adolescent period must, as regards its complete description, be left to a subsequent 

 memoir, material for the completion of this part being at present wanting. 



