ACTINIARIA 147 



times takes place. Concerning the former they seem to be almost exclusively' mesogloeal in Cribrinopsis; 

 in Urticina they variate, as I wiU show below, from ectodermal to meso-ectodermal or ecto-mesogloeal, and 

 this even in the same species. In the other genera these muscles are ectodermal or meso-ectodermal as in 

 Cribrina elegantissima and spetsbergensis. It is exactly the same with the radial muscles, though they are 

 never as much enclosed in tlie mesogloea as the longitudinal muscles of the tentacles. Commonly they are 

 ectodermal, in the genera Cribrina and Urticina ectodermal or meso-ectodermal, in the latter case they agree 

 with those of Cribrinopsis. In Urticina they variate from ectodermal to meso-ectodermal in the same species. 



Concerning the distribution of the reproductive organs in the older mesenteries, the genera Isotealia 

 and Urticina differ from the other genera. In Urticina, according to its age, only the 6 first pairs of mesen- 

 teries, or the 10 or 20 oldest mesenteries are sterile (compare below), while in Isotealia the reproductive organs 

 first appear on the mesenteries of the third cycle. The other genera have reproductive organs, as far as we 

 know, even in the mesenteries of the first order which remain fertile. 



The arrangement of the mesenteries as a genus character is of more secondary significance, as it 

 varies considerably especially in certain genera, even in species such as Cribrina and Cribrinopsis. On the 

 other hand the mesenteries are in the other genera more typically, hexamerously arranged, while the genus 

 Urticina shows decamerism. Especially concerning the latter genus the question has been raised, whether 

 the decamerism may be used as a genus-character. Mc. Murrich (1901 p. 21) namely declares; "to establish 

 a genus on its decamerism seems to me .... to place it on an exceedingly insecure foundation." He founds 

 his statement, for one, on an information by Verrill that "many Urticina crassicornis are hexamerous, many 

 others decamerous, some octamerous and a few irregularly or unequally developed on opposite sides." If 

 that really is so, the decamerism is here certainly worthless as a genus-character. At present I, however, 

 much doubt that Ver rill's identifications of the genus have always been correct. As we wiU see from the 

 following it is very difficult, without the most careful investigation, to distinguish the genus Urticina from 

 another genus, Cribrinopsis, and young specimens of Urticina likewise from Cribrina. I for my part have 

 almost always found Urticina decamerous (only a single time octamerous), though irregularities occur, so 

 that not all mesenteries of tlie same cycle may be developed. There is, besides, nothing astonishing in this 

 that the arrangement of the mesenteries in an early period of its hfe displays a variation, as Urticina during 

 its development passes through a hexamerous stage. To my mind the decamerism may be used as a genus- 

 character to Urticina, though with a certain restriction. Decamerism, octomerism and hexamerism princip- 

 ally may be used as genus-characters. Certain genera (and species) namely have a more constant mesenterial 

 arrangement than other genera, wherefore the arrangement is usable here as a systematic character, while 

 other genera show so great a variation in the grouping of their mesenteries that the mesenterial arrangement 

 is useless for systematic purposes. Of course we must leave out of consideration accidental defects of the 

 mesenteries causing any kind of disorder to the typical arrangement. There can, for instance, be no doubt 

 that the arrangement of the 8 " Edwardsia-iaesenteries" is of great systematic importance, though in the 

 Milne-Edwardsinae only 7 mesenteries exceptionally occur (p. 64 compare a similar suppression of mesenteries 

 in a young Peachia p. 105). Finally it must be proved in each special case if the decamerism, the octomerism 

 etc. is due to regeneration, in which case it is of no systematic importance, as I have before pointed out 



(1914 p. 63). 



19* 



