STAUROCALYPTÜS ENTACANTHUS. 193 



through the dermal layer, may be 3 mm. wide, and that in a 

 position not more than 100 mm. distant from the uppermost end 

 of the specimen. In the interspaces between the apertures the 

 ectosomal layer is in tolerably close contact with the choanosomal 

 surface. 



The gastral surfoce, so far as this extends in the specimen, 

 is entirely lined with a well-developed endosomal latticework, 

 which is for the most part very distinctly set apart from the 

 underlying choanosome and covers up all the excurrent canalar 

 apertures (up to 3 mm. dia.), in a measure concealing these from 

 view. Its appearance is not unlike that of the same structure in 

 aS'. affinis, in which species, however, it is developed in l^ut small 

 irregular patches and not all over the gastral surface. The 

 endosomal beams, the main suj^port of which is afforded by 

 strands of hypogastral diactins, are of a moderate strength and 

 inclose small, irregularly shaped meshes (mostly i-f mm. wide), 

 which are open and not filled in by a continuous lacework of 

 gastralia, unlike those in S. fasciculatus but quite like the same 

 of Acanthascvs cactus (see PI. XL, fig. 16). Tliis endosomal 

 latticework, proba1;)ly developed over the entire gastral surface and 

 leaving none of the excurrent canalar apertures quite freely open, 

 may be regarded as one of the distinctive features of the present 

 species as contrasted with many closely related forms in the same 

 genus. 



Another noteworthy, and I think characteristic, point in 

 the features of the gastral surface is the fact tliat numerous, 

 moderately strong oxydiaetins project freely ])eyond the surface, 

 mostly to a length of 4 or 5 mm. There is no I'ule as to the 

 direction of the projecting needles, which occur over the entire 

 gastral surface so far as this is represented in tlie fragment, but 



