AND RELATED GENERA, SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 53 



ture of the disc is in the ridges almost as well developed as the longitudinal musculature 

 on the inner side of the tentacles, but in the furrows it is considerably feebler. 



The ectoderm of the oesophagus shows the usual structure, but in addition numerous 

 thick- walled nematocysts (length 43-48 m, breadth 5 m), and in fair numbers spirocysts 

 of the same appearance as in the body-wall. The mesoglcea, which is very thick, agrees 

 in structure with that of the body-wall. 



The arrangement of the mesenteries is shown by the accompanying schematic 

 fig. 2 ; 6 pairs of mesenteries of the first order, with 2 pairs of symmetrical, direc- 

 tive mesenteries are present. While all the primary exocoels are reduced, so that no 

 mesenteries could be formed in them, each of the 4 lateral endocoels contains a pair 

 of mesenteries of the second order, in which, however, the longitudinal muscles face 

 outwards, as in the directive mesenteries. The mesenteries of the first and second 

 order, all of which are perfect, are thus 20 altogether; of these 16 are grouped 

 in 8 pairs with longitudinal muscles as the directive mesenteries, owing to the develop- 

 ment of the mesenteries of the third order in the secondary endocoels, and the last 4 

 mesenteries are unpaired at the sides of the directive mesenteries. The arrangement of 

 the 20 oldest mesenteries thus agrees completely with the corresponding condition in 

 Halcurias. In the endocoels of the second order we find mesenteries of the third, 

 fourth, and fifth order. The mesenteries of the third order (8 pairs) are perfect, and 

 have longitudinal muscles typical of the Actiniaria (muscles facing inwards). The 

 mesenteries of the fourth order are unequally developed. On one side of the 

 mesenteries of the third order, between these and the mesenteries of the second 

 order, there is an imperfect mesentery of the fourth order, but on the other side a 

 pair, consisting of one imperfect mesentery, lying nearest the mesenteries of the 

 third order, and a perfect mesentery. Between the latter and the mesentery of the 

 first order there is an imperfect mesentery of the fifth order. The arrangement of 

 the mesenteries is thus : 6 — 4 — 8 — 8 + 8 unpaired — 8 unpaired ; thus in all 26 pairs, 

 + 8 unpaired of the fourth and 8 unpaired of the fifth order, or 68 mesenteries. 

 While this is typical, the development probably proceeds somewhat further in some 

 ways in very old specimens. 



The mesenteries are somewhat thick, owing to the thickness of the mesoglcea. The 

 longitudinal musculature is fairly well developed, though not so much as in Halcurias. 

 In the distal part the folds are thick, but grade towards the proximal end to a weak, only 

 a little condensed, pennon-like region (figs. 7, 8, PL IV.), which approaches the body- wall 

 and fuses with the well-developed parietal parts of the longitudinal musculature. The 

 parietobasilar musculature is narrow but fairly well developed (figs. 7, 8, PL IV.), and 

 goes far out towards the distal end. There are no separate basilar muscles, though in 

 the foot end the muscles pass in a transverse direction, also on the longitudinal muscles' 

 side of the mesenteries ; but these muscles which run transversely are continuations of 

 the longitudinal musculature, which at the base of the column bend in a transverse 

 direction (cf. p. 60, Ilyanthopsis elegans). 



