200 



ACTINIARIA 



Anatomical description: The ectoderm of the column is almost lost, only in the uppermost 

 part there are some fragments proving it to be thin. The mesogloea is very thick, of the same consistency 

 as in Actinostola, fibrillar with scattered, small, often round cells. The endodermal circular muscles are weak. 

 The sphincter is comparatively weak, the meshes placed in groups showing a tendency to transversal strati- 

 fication; it is the strongest on the upper part and gradually becomes weaker downwards. It occupies only 

 one part of the breadth of the mesogloea and seems to be wholly separated from the entodermal circular 

 muscles (textfig. 189). The ectoderm of the tentacles is high and provided with numerous typical nemato- 

 cysts, which are rather broad in comparison to their length (26 34 x 4 5/*), and with very numerous 

 spirocysts of very variable size (from 19 x 1,5 2 // to 67 x 7 //). The ectodermal longitudinal muscles are 

 not strong, the folds are rather low and commonly not branched, but rather close (textfig. 191). There is 

 no great difference in the development of the muscles on the adaxial and abaxial sides ; at the base the muscles, 



however, are a little stronger on the 



Textfigs. 189 191. 



Epiparactis dubia. 



Transverse sections of sphincter (fig. 

 189), of part of the oral disc (fig. 190) 



adaxial side, and the mesogloea likewise 

 a little thicker on the abaxial side of 

 the base. Still we cannot speak of basal 



and of part of a tentacle (fig. 191) thickenings of the mesogloea. The meso- 

 cm: circular mnscles of the column. 



gloea of the tentacles is commonly rather 

 thin. The radial muscles of the oral 

 disc is also ectodermal (textfig. 190), 

 in its outer part stronger than in the 

 tentacles and provided with rather high, 

 close folds, of about the same dimen- 

 sion at the insertions of the mesenteries 

 as at the ridges. The ectoderm of the 

 actinopharynx contains numerous ne- 

 matocysts , 24 41 X 3,5 5 p in size. 

 The mesenteries are hexamerously arranged (6 -f- 6 + 12 + 24 + an imperfect fifth cycle). Only 

 the 6 first pair are perfect. The imperfect mesenteries have been examined in 5 / 6 of the animal. The mesen- 

 teries of the fifth cycle were weak and generally present only in the outer compartments viz. beside the me- 

 senteries of the first and second orders, sometimes they are lacking in some of those, sometimes they are 

 also developed in the inner compartments beside the mesenteries of the third order. Both mesenteries of 

 a pair were not developed in conformity with the Actinostola-rule. The mesenteries form thin lamellae. 



Fig. 189 



Fig. 190 



Fig. 191 



Genus Pycnanthus Me. Murr. 



Diagnosis: Paractiidae with well-developed, enlarged pedal disc. Column smooth, without tuber- 

 cles, in contracted state low and thin, sometimes with more or less distinct longitudinal ridges in the upper part. 

 Upper parts of the column capable of involution. Margin tentaculate, distinct, not lobed. Sphincter weak or 

 well developed. Tentacles short, only half as numerous as the mesenteries, the inner considerably stronger 



