The Pluteus of Laganum sp. 



137 



the large, darkly stained nuclei bulge out prominently into its cavity. 

 The lumen is largest at the right and left sides of the gut, while in the 

 region of the mid-line, dorsal and ventral to the stomach, the entero- 

 coele is so compressed that the walls often lie in actual contact. In 

 many of the specimens, moreover, it does not completely surround the 

 gut, but is interrupted by a strand of cells connecting the stomach with 

 the aboral wall of the embryo. This commences slightly to the left of 

 the midline at the anterior end of the stomach, and gradually curves 

 to the left side as it approaches the posterior. That this mesentery-like 

 structure is not a permanent one is shown by the fact that in many of 

 the specimens, especially those showing a slightly more advanced stage 



FIG. 20. Median sagittal 

 section. %a.e., anterior en- 

 teroccele; g, gut; h, hy- 

 droccele; Z 5 , lumen of lobe 

 5 of hydrocoele; m, open- 

 ing into amniotic cavity: 

 sp., spine; t 6 , tentacle 5. 

 X 233. 



20 



of development, it is entirely lacking and the lumens of the right and 

 left lobes of the enterocoele communicate freely around the dorsal 

 side of the stomach. Figure 20 shows a specimen in which the tissue 

 is especially well developed. 



Considering the fact that the enterocoele is as yet undivided, the 

 hydrocoele has attained a remarkably advanced stage of development. 

 The ring is well formed, and in some cases closed. It occupies a posi- 

 tion ventral to the anterior portion of the gut, in the mid line. It is 

 composed of small, cuboidal cells with dense, rather opaque cytoplasm 

 and conspicuous nuclei, and contains a small but definite lumen. There 

 are 5 prominent lobes which have already differentiated into the well- 



