158 



ACTINIARIA 



(compare the table). To judge from the grouping of the mesenteries the tentacles are commonly decamerously 

 arranged, though a hexamerous arrangement also seems to occur. They are cylindrical or in more extended 

 state a little conical, more robust than in Urticina, and in contracted state irregularly folded or longitudinally 

 sulcated with numerous transversal folds and with a distinct opening in the apex (PI. 3 fig. 7). The outer ten- 

 tacles are considerably smaller than the inner (about half as long). At the entrance to the siphonoglyphes there 

 are conspicuous gonidial tubercles, the siphonoglyphes are well marked and furnished with well developed 

 aboral prolongations. The actinopharynx is long and has longitudinal folds in great numbers. 



Anatomical description: The ectoderm of the column is high and contains nematocysts 17 — 25 

 X 2 /^ in size, in the specimen from Finmark the nematocysts were a little longer (23 — 30 X 2,5 (i). The 

 verrucae seem to be of the same structure as those of Urticina. The mesogloea is thick and contains rather 

 numerous, protoplasma-poor cells. The endodermal circular muscles is rather well developed. The sphincter 

 is strong, pinnate or palmate. Two examined species were furnished with such a sphincter, as the textfig. 

 165 (specimen from Finmark) shows, viz. with a strong main lamella, tliickening inwards; in a specimen 

 (from the Corea strait) the sphincter was distinctly palmate without a main lamella. A specimen from Kola, 

 Chewanna had a sphincter with a short and thick main lamella (textfig. 166). 



The ectoderm of the tentacles is high and contains numerous nematocysts, which in the apex reach 

 a size of 34 — 70 X 2 — 2,5 (3) //. They are of the same length as those of the actinopharynx, but the latter is 

 considerably broader (about double as broad). The size of the nematocysts and spirocysts from part of the 

 material is given in the following table, in which also a survey of the variation of some other organs has been 

 included (p. 160). 



The longitudinal muscles of the tentacles are very strong, almost entirely enclosed in the meso- 

 gloea, and separated from the ectoderm by a commonly thick mesogloeal lamella. Towards the ectoderm 

 the mesogloea projects into fine, sometimes ramificating off-shoots, between these there are sometimes (al- 

 ways?) solitary muscle-fibrils, never forming any coherent layer. These muscles are considerably thinner than 

 those enclosed in the mesogloea and are probably in a state of reduction. The muscle meshes are in contracted 

 tentacles radially extended, they are sometimes rather large (textfig. 167 specimen from Finmark) but 

 commonly densely packed together (textfig. 168). Still denser meshes may occur. The radial muscles of the 

 oral disc are not so much enclosed in the mesogloea as the longitudinal muscles of the tentacles, but the muscles 

 here may commonly be designated as ecto-rnesogloeal. Between the insertions of the mesenteries they have 

 commonly the appearance as the textfigure 169 shows, sometimes the meshes are smaller. At the insertions 

 of the mesenteries the muscles are weaker and commonly not so enclosed in the mesogloea as it is in the middle 

 part between the insertions, and here it is sometimes chiefly ectodermal. The ectoderm of the actinopharynx 

 contains numerous nematocysts of about the same length as those of the tentacles, but in breadth they are 

 considerably larger. The size variates between 36 — 67 (70) x 3,5 — 5 //. 



The number as well as the arrangement of the mesenteries variate. The most closely examined spec- 

 imens had 40 or about 40 pairs of mesenteries decamerously arranged (10 + 10 + 20; 10 + 10 + 20 + i). 

 In the two largest specimens the mesenteries were hexamerously arranged, in the specimen from Behring's 

 Sea the number of mesenteries was 46 (6 + 6 + 12 -|- 22), on one side all the mesenteries of the 4th cycle 



