ACTINIARIA 



77 



basal part to the spiral thread. The uppermost part of the column is provided with distinct, though weak 

 longitudinal muscles, forming a continuous layer in the tract where the spirocysts are common, but soon 

 disappearing. A nerve-layer with nerve-cells is present and is the most developed in the uppermost part of 

 the column. The mesogloea of the column is composed of alternate layers of longitudinal and circular fibrillae. 

 The endodermal circular muscles form no special sphincter, but are rather well developed; in the proximal 

 part, close by the proximal end, they are a little stronger than in the other parts. The muscle-folds are besides 

 of different appearance, according to the state of contraction. The tract, corresponding to the region of 

 the tentacles and the oral disc in other Actiniaria, is provided with a remarkably high ectoderm, several 

 times higher than that of the column. The outer parts of this ectoderm consists, as a transverse-section 

 through the oral disc shows (textfig. 93), almost exclusively of extraordinarily numerous, very closely packed 

 spirocysts, 19 36 p long; only exceptionally nematocysts appear, n 14 // long. The inner parts of the 

 ectoderm are very deeply stained with the carmine of borax. Here we see the mother- 

 cells of the spirocysts, and spirocysts in development. The nerve-layer is not very 

 distinct, on account of the immense number of stinging capsules. The radial muscles 

 of the oral disc are weak and its mesogloea thin. The ectoderm of the actinopharynx 

 is several times thicker than the mesogloea, here and there sending out irregularly pla- 

 ced tongues towards the lumen of the actinopharynx. 



The nematocysts of the actinopharynx ectoderm are broader in the basal 

 end than in the distal one, and the basal part to the spiral thread is perceptible; their 

 size is about 24 26x4^. Also in the actinopharynx there are longitudinal muscles, 

 though weak, but distinct, especially in the aboral part. The actinopharynx is provided Textfi g- 93- Limnactinia 



laevis. Transverse section 

 with several longitudinal ridges and furrows. I have not observed any siphonoglyphe. of oval disc. 



The mesenteries are partly perfect with longitudinal pennons, reproductive organs and filaments, 

 partly imperfect without such organs. The number of perfect mesenteries variates from 8 to 10. In three 

 examined specimens only the 8 "Edwardsia-mesenteries" were perfect (textfig. 94 95); in two larger spec- 

 imens, provided with well developed reproductive organs, also the fifth couple was perfect (textfig. 96) ; thus 

 the number and the arrangement of the mesenteries were in conformity with those of Pentactinia (Carl- 

 gren 1900). Four (2 dorsolateral and 2 ventrolateral) or two (ventrolateral) imperfect mesenteries together 

 with the 8 resp. 10 perfect " Edward sia-mesenteries" form the first cycle of six pairs of mesenteries. An im- 

 perfect second cycle of weak mesenteries, not projecting over the surface of the endoderm, is present. In 

 one specimen only two pairs of mesenteries, one in each dorsolateral exocoel, were developed; in the other 

 specimens there were 4 pairs in the dorsolateral and the lateral ^xocoels. In the ventrolateral exocoels I have 

 never found any mesenteries. The arrangement of the muscles is the same as in other elongated Athenaria. 

 The longitudinal pennons are strong, the high folds, however, rather few, in the upper part of the repro- 

 ductive region about 9 15; they are mainly branched in their outer parts. The parietal muscles as well as 

 the muscles of the imperfect mesenteries are provided with short and coarse folds. The expansion of the 

 parietal muscles on the column is considerable and often extends far sideways from the folds of the parietal 

 muscles. Below the pennons the parietal muscles are as usual more elongated, form no folds and are attached 



