60 PROFESSOR OSKAR CARLGREN ON THE GENUS PORPOMA 



Halcurias pilatus he had found longitudinal muscles in the ectoderm of the body-wall. 

 Though I have not examined this species, I consider it possible that the same condition 

 prevails there as in Ilyanthopsis — in other words, that the longitudinal muscles are 

 no other than thickened, basal parts of the ectoderm cells. The mesogloea of the 

 body-wall is very thick, and provided with extremely numerous cells with bipolar, 

 irregularly prolonged ends. In the inmost part of the mesogloea there are closely 

 packed fibrillar folds arranged as fig. 1, PL IV. shows. The sphincter I have not 

 closely examined, but I think it probable, from an external investigation, that this is 

 completely wanting, as Wassilieff states. The entoderm is somewhat broader than 

 the ectoderm, and thicker in the middle between the insertions of the mesenteries than 

 at the sides. Wassilieff has described the structure of the tentacles and emphasised 

 the longitudinal muscles of the ectoderm. The ectoderm contains very numerous 

 spirocysts of varying size, and also numerous thick-walled nematocysts (length about 

 36 m). The longitudinal musculature of the ectoderm is comparatively weak. How 

 far fine outshoots run out from the mesogloea into the ectoderm, as is stated by 

 Wassilieff, I am unable to determine ; it seems to me that these outshoots are 

 nothing else but the thread-like basal parts of the ectoderm cells. The ectodermal 

 nematocysts and spirocysts of the oral disc agree with those in the tentacles, but are 

 not so abundant. The ganglion and nerve layer is also well developed here, as in the 

 tentacles. The ectodermal radial musculature is strong, especially in the ridges, and 

 the muscle folds just as high as, or higher than, the epithelial parts of the ectoderm. 

 The inner parts of the muscle folds show a tendency to be mesogloeal in the ridges 

 (fig. 3, PL IV.). The structure of the oesophagus agrees with that of the body- wall. 

 The mesenteries are all complete, corresponding in number to the tentacles. 



As Wassilieff states, the musculature is very weak, and only somewhat more 

 strongly developed where the mesenteries join the body-wall (fig. 1, PL IV.). Here we 

 can distinctly distinguish the muscles, both the longitudinal and the parieto-basilar, 

 which are weak. On the very thin mesenteries there is no trace of protuberance of 

 the longitudinal muscles. In longitudinal sections through the mesenteries and trans- 

 verse sections through the base it looks as if the basilar muscle occurred as a weak 

 fold of the musculature of the mesenteries. But specially differentiated basilar muscles 

 do not seem to be developed, for the transverse layer of muscles near the base on 

 the side of the mesenteries where the longitudinal muscles are, is formed by 

 the latter muscles, bending almost at a right angle a little way from the base, 

 thus forming what may be called pseudo-basilar muscles (fig. 7, PL IV.), the same 

 condition as in Porponia, as I was able to determine from preparations of the 

 mesentery. With regard to the arrangement of the mesenteries, it is impossible to 

 determine whether all of them are equally developed. So much can be said, however, 

 that in the specimen I have examined, in addition to the directive mesenteries with 

 longitudinal muscles on the outer side, the other mesenteries seem to be arranged in 

 pairs with the longitudinal muscles on the inner side. The Halcurias stage, or rather 



