CERIANTHARIA. 



67 



very soon disappears, so that only the median streak remains, in the form of a craspedion. The craspedia 

 are to begin with and for the greatest part of their course of the same structure as the median streak, 

 save that the ectoderm of the craspedion region is lower, less prominent and that the groove between 

 the parallel bands of the spirocyst-glandular tract is not so deep. In the most aboral portion these 

 bands gradually coalesce. The ciliated groove between them disappears, so that the craspedium now 

 presents itself as a uniform structure with epithelium of the same height, which in transverse section 

 shews as a broad flat layer. Here in particular the craspedia are smaller in cross-section than the 

 median streak in tlie ciliated tract region. In the most aboral portion of the craspedion we now 

 find in addition to spirocysts very large tliick-walled nenuitocysts especially near the limit of tlie 

 ectoderm. These nematocysts have the same appearance as in the entoderm of the mesenteries. They 

 present much the same appearance in Arachnactis albida. 



In C. lloydii the craspedia are fairly broad adorallv, but taper off in the aboral direction and 

 sink more and more into the entoderm, so that in transverse section they present a small oval with 

 the longer axis in the mid-plane of the mesentery. The craspedia contain in the aboral part very 

 numerous large thick-walled nematocysts, which gives the craspedia a more mixed character. 



In Botniciiidifcr iwrvcgicus the craspedia are broad and consist chiefly of gland cell.s, whilst 

 nematocysts are more sparse. 



In P. iiiaiia the craspedia are narrow and contain a very thin sprinkling of nematocysts, if 

 any at all. ; 



The craspedia in Ccriaiithcopsis americanus are very broad throughout their whole course and 

 except in the lower part they contain, in addition to supporting cells, principally thiu-walled nemato- 

 cysts and homogeneous gland cells. The uniscular layer is quite well developed. In the most aboral 

 part is found on the metamesenteries of the first cycle a thread-shaped craspedoneme (compare 

 p. 26). This shews a different structure. Whilst the homogeneous glands cells are still to be found, 

 mimerous granular gland cells now present themselves. The thin-walled nematocysts disappear almost 

 entirely, and in their place thick-walled nematocysts are now met witli in very large numbers. The 

 muscular layer too is distinctly feebler than in the upper part of the craspedion. 



As we have seen, not only tlie ciliated tract region and the cnido-glandnlar tract 

 but also the craspedion region may tlius form craspedonemes, though in the latter case 

 they are more diminutive and consist of a single thread only. The craspedoneme in the craspedion 

 region of C. americanus is no doubt suggestive of the structure of the cnido-glandular tract, by the 

 presence of thick-walled nematocysts though the accordance is not complete: still as in C. lloydii and 

 Arachnanthus oligopodus thick-walled nematocysts also occur in the under (aboral) portions of the 

 craspedia. We shall hardh- be going too far in regarding the single craspedoneme in the most aboral 

 part of the mesenteries of C. americanus as a differentiation of the craspedion. For if we regard this 

 craspedoneme as a differentiation of the cnido-glandular tract, we should have in C. americanus on 

 protomesenteries 2 and on the longer more de^•eloped metamesenteries of the first cycle two portions 

 of the gnido-glandular tract, separated from each other by a very long craspedion region, a circum- 

 stance very hard to explain. 



