ACTINIARIA 



237 



Further distribution. The North Sea, British Islets, Shetland Isl. 



Exterior aspect. The exterior of this species has been described before by various authors, where- 

 fore further discussion of it is unnecessary. In some specimens the column is rather thick in preserved 

 state. Concerning the pedal disc (compare p. 233). 



Anatomical description: The anatomy of this species has also been described by myself (1893), 

 by Verrill (1899) and by Me. Murrich (1911). Me. Murrich's account of the organisation agrees with 

 mine, except in some small details; on the other hand, Verrill's account differs from mine in some important 

 characters, as I have mentioned above. On some points I will, however, complete my earlier observations. 

 Concerning the stinging capsules, there are in the tentacles two kinds of nematocysts, partly typical rib-like, 

 smaller ones (a), partly larger, broader in the basal end, and sometimes provided with discernible basal part 

 to the spiral thread (b). In the actinopharynx we also find two kinds of nematocysts, partly typical (a), 

 partly with discernible basal part to the spiral thread (c). In the distal part of the tentacles the largest nema- 

 tocysts are found. Eight more closely examined specimens show a good agreement in the size of the stinging 

 capsules, as shown by the following list. 



The with * designated stinging capsules also contained capsules from the proximal part. The nemato- 

 cysts of the column are small, in an examined specimen their size was 14 17 X 1,5 p. 



Concerning the mesenteries, I have before (1893) put forth that the youngest mesenteries are developed 

 according to the Actinostola-rule. This rule also seems to be valid as far as the older mesenteries are concerned, 

 it is especially distinct in the first cycle of imperfect mesenteries. It is true that these latter seem to be 

 equally developed in transverse sections, but their insertion on the pedal disc shows that both mesenteries 

 of one and the same pair are of different size (textfigure 210). The Actinostola-rule is, however, not so distinct 

 here, because the development of the mesenteries, after the appearance of the 6 first pairs of mesenteries, is 

 not the typical one. Instead of a development of 6 pairs of mesenteries of the second order, common in the 

 Actiniaria, 10 or 12 pairs have arisen which are all or for the greater part perfect, like the mesenteries of the 

 first order. On textfigure 210 we see that two pairs of the second order and 3 pairs of the third order corres- 

 pond to each primary exocoel. Of these latter, which form the first cycle of imperfect mesenteries, the 

 weakest mesenteries in two adjacent pairs are facing each other- to judge from the extension of the mesen- 

 teries on the pedal disc and stand nearest to the interjacent pairs of the second order (designated with II). 



