286 ME. J. E. DTJEEDElSr OlS" 



The various stages leading to this are represented in fig. 14, 

 taten from a single oblique section in which the upper part is 

 more aboral than the lower. Traces of the two sinuses are still 

 seen in the latter region, and on the right side the last evidence 

 of the sulcar canal ; while, in the upper half of the figure, the 

 V-shaped canals, embracing the free edge of the mesenteries, are 

 distinct. This latter condition holds for all the mesenteries as 

 far as their lower termination. 



A vertical section, such as would be obtained along the line 

 3c-y in fig. 12, is represented in fig. 15 (PL 19), taken from one of 

 the youngest larvse. The archenteron opens into a shallow central 

 chamber, from which two canals radiate, one to the left and the 

 other to the right. Histologically the floor of the chamber differs 

 considerably from the undiflJerentiated tissue below, more nearly 

 resembling the endodermal lining of the archenteron. Vertical 

 sections of one early larva show this chambered region very 

 distinctly ; some (figs. 10, 11) with the right side terminating 

 blindly, and others (fig. 9) with a canal-like continuation. The 

 precise outline of the diverticulum to tte left in fig. 15 is largely 

 deduced from the various transverse sections, as only isolated 

 parts are recognizable in the actual vertical preparations. 



The boundary of all the canals and spaces is regular and 

 smooth, and cilia can be distinguished, not, however, so thickly 

 disposed as in the archenteron. Everywhere at its origin the 

 limiting layer is in continuity with the archenteric lining, and 

 throughout bears a close resemblance to it. The cells constituting 

 it are more or less separable from the rounded, vacuolated cells 

 below, being columnar and filled with finely granular, protoplasmic 

 contents ; oval nuclei are much more numerous than elsewhere, 

 and are arranged in a somewhat regular layer. 



Throughout the best-preserved larvae the margins stand out 

 prominently in sections stained in boras-carmine, owing to the 

 greater abundance of nuclei which readily take up the stain. 



In sections the vacuolated tissue, both central and peripheral, 

 most closely recalls the parenchymatous tissue of plants. The 

 cells are somewhat polygonal in outline, their boundaries being 

 extremely thin, and the contents have almost entirely disappeared. 

 Here and there a nucleus occurs applied to the walls, and zoo- 

 santhellae are distributed throughout. No objects at all sugges- 

 tive of yolk-granules are present in any of the stages. The tissue 



