124 



acontia, filamentar nematocysts, ridged stomodoea, etc. — better 

 than the view derived from the ontogeny of the few types that 

 have been descnbed. In effect, it seems to me that this is an 

 excellent instance where natural selection may have acted on 

 the developing polyp, in which the whole of the cavity of the 

 body was open to the water, the nematocysts serving for its 

 protection. With the formation of the stomodoeum in the adult 

 the main function of the endodermal and filamentar nematocysts 

 would be gone. It would then obviously be of no small advan- 

 tage for the formation of the stomodoeum to be hurried on as 

 early as possible, so that all trace of the original condition would 

 only remain in the degenerate endodermal nematocysts and in 

 the filamentar ones, which I cannot consider to have at the pre- 

 sent time any adequate function to perform. 



2. TROCHOCYATHUS RAWSONII. 



The polyp extends down outside its cup for about a third of 

 its height. It has 24 tentacles, the bases of which are confluent, 

 and each crossed by a pair of mesenteries. The longitudinal 

 muscles extend into them, and in contraction of the polyp com- 

 pletely introvert them from the base. 



There are 48 mesenteries, septa IV. being exocoelic ; 24 

 of these reach the stomodoeum, which is ridged over their attach- 

 ments precisely as in FlabelLum. Their filaments are of the 

 usual form, but have no massed loops, corresponding to the 

 acontia ; their lower ends though are free, and may be capable 

 of some degree of extension. Nematocysts much larger than 

 in Coenopsammia, and with about 16 coils are present, parti- 

 cularly in the lower halves of the filaments, which are often 

 almost choked with them. 



The endoderm forms pads under the mesenterial filaments, 

 but is not especially thickened over the generative organs. Scat- 

 tered throughout it e\^erywhere, but more numerous over the 

 muscular bands, are a series of deeply staining bodies. A fe\* 

 of these, round in shape, deeply and evenly stained, are evidently 

 fat globules. Others, more granular in appearance, and less 

 regular in shape, appear to be simply food patches in the 

 Plasmodium, which forms the endoderm. Nematocysts are 

 present in patches on the septal sides ; they are of about the 

 same size as the filamentar nematocysts, and almost in the same 

 condition as in Flabellum, but with the thread less distinct. 

 Testes occur in all the mesenteries. They are only one follicle 

 thick, but in the larger mesenteries form great masses, deep 

 down in the calicle, almost completely filling up the interseptaJ 

 spaces. 



