154 



ACTINIARIA 



d + and d o = directives with and without reproductive organs. 



The longitudinal muscles of the tentacles are always strong, sometimes thej' form very high folds. 

 These latter are closely packed and of a palisade-shaped appearance, though thej' are more or less dicho- 

 tomously branched. The main part of the muscles is ectodermal, here and there small parts are however 

 enclosed in the mesogloea at the base (textfig. 160), as well as higher up on the folds (textfig. 161). I have 

 examined specimens from all localities and all show a similar appearance of these muscles, with the exception 

 of a specimen of Kwietniewski's Leiotealia (compare Urticina felina p. 175) which had ectodermal muscles. 

 The muscles of the tentacles are also meso-ectodermal. The radial nmscles of the oral disc in their arrange- 

 ment resemble the longitudinal muscles of the tentacles. Here the folds are, however, commonly more in 

 connection with each other in the middle part between the insertions of the mesenteries, as the textligure 

 162 (St. 67) shows. At the insertions of the mesenteries the muscles are much weaker. The ectoderm of the 

 actinopharynx contains very numerous nematocysts (compare the table). 



The number of the mesenteries seems to variate considerably, as shown by the table. The smallest 

 number in an adult individuum was 18 pairs, the largest 51 pairs. The pairs of mesenteries were besides some- 

 times a little more numerous on one side than on the other side. The mesenteries are evidently hexamer- 

 ously arranged, though it was to be expected that with such a variable number of mesenterial pairs as 18, 

 32, 40 and 48 etc. (compare the table) also the cardinal number might be liable to variation. The cause of 

 the seemingly great variation in the number of mesenteries is connected with the miscarrying of the mesen- 

 teries of the last order in certain exocoels, and sometimes with the occurrence of some other irregularities in 

 the development. The arrangement of the mesenterial pairs in the specimen from Finmark for inst. was 

 6 -|- 6 + 6 = 18. Issuing from one pair of directives the mesenterial pairs 2, 4 and 6 of the third cycle 

 were not developed on any side of the directive plane. The arrangement of the mesenteries in the specimen 

 with 32 pairs (from St. 67 "Michael Sars") was 6 + 6 -j- 12 + 8. Of the compartments between the 

 mesenteries of the first and second order one was typically developed with one pair of the third and 2 pairs 

 of the fourth cycle, 5 compartments were devoid of all mesenteries of the fourth cycle, and 6 compartments 

 contained one pair of the fourth cycle instead of two. In the specimen with 40 pairs the number of mesenterial 



