ACTINIAKIA 221 



ill the weaker of the sterile pairs both mesenteries of one and the same pair sometimes are distinctly different 

 in size. This is probably in reality still more commonly the fact than may be concluded from the figure, 

 as an attempt of ascertaining their size meets with the same difficulties here as in S. tuberculata. 



The arrangement of the mesenteries thus is distinguished from that of the preceding species by 

 its being more irregular, and this irregularity probably (at least in some cases) appearing in all 6 primary 

 exocoels, not only in 4 as in S. tuberculata and ingolfi. 



Alternating with the sterile mesenteries there are weaker, fertile mesenteries (on figure 207 not drawn, 

 on the figure A , reproduced specimen, the reproductive organs (in the weakest mesenteries) have not yet been 

 developed). These fertile mesenteries also here arise at the limbus of the pedal disc and grow more or less 

 upwards, they are the least developed in specimen A, the most in specimen B, in which they reach the region 

 of the actinopharynx. 



The longitudinal muscles of the stronger mesenteries form no pennons, but are about equally expanded 

 over the whole surface of the mesenteries. The folds are rather numerous and of ordinary height, they are 

 weakest at the column. On the directive pairs, the folds of which are stronger, the innermost part displays 

 the highest folds. The parietobasilar muscles are in their lower part broad, but rapidly narrowing and proceed 

 upwards as a thin lamella, but do not reach the sphincter. They are not, or somewhat, folded as in the preced- 

 ing species. The basilar muscles are rather well developed (textfig. 205), though not strong, as they are sup- 

 ported by a thick mesogloea. Oral and marginal stomata are present. Whether the latter, which are often 

 very small, are present on all mesenteries, I have not been able to determine. The ciliated streaks are well 

 developed, and the filaments in the region of the ciliated streaks strong. The sterile mesenteries bear filaments, 

 the fertile are devoid of such. The species is dioecious. 



Genus Actinostola Verr. 



Diagnosis. Paractiidae (Actinostolinae) with the body either short, cup-like, in the proximal part 

 small, in the distal broad, or long cylindrical. Column mostly thick, firm, slightly rugose or almost smooth 

 or with flat tubercles of mesogloeal thickenings, unlobed in the distal part, without verrucae, acrorhagi and 

 fossa. Sphincter in comparison to the size of the body usually rather weak, so that the body-wall, at the con- 

 traction of the animal, for the greater part cannot cover the tentacles. Tentacles short, the inner consider- 

 ably longer than the outer, about as numerous as the mesenteries, hexamerously arranged, in contracted 

 state almost cylindrical, irregularly rugose, sometimes with mesogloeal thickenings at the base of the outside, 

 with mesogloeal longitudinal muscles. Radial muscles of the oral disc mesogloeal. Two well developed siphono- 

 glyphes. Numerous perfect mesenteries, hexamerously arranged. The two mesenteries in one and the same 

 pair, from the third or the fourth cycle, irregularly developed but as a rule outlined, so that the mesentery, 

 which generally turns its longitudinal muscles towards the preceding cycle of mesenteries, is more developed 

 than its partner. Retractor of the mesenteries diffuse. Parietobasilar and basilar muscles strong. Mesenteries 

 of the first and the second order sterile. Reproductive organs first arise on the mesenteries of the third cycle. 

 The fertile mesenteries have filaments. 



