ACTINIARIA 167 



mesogloeal off-shoots here evidently takes place rather late. Probably we here namely have to do with two 

 different modes of development of the mesogloeal muscles. On the section, reproduced in the textfigure 

 174, the rather large, at last mesogloeal meshes may gradually have passed into the mesogloea, while on the 

 section, reproduced in the textfigure 173, the peripheric end of the mesogloeal offshoots have fused with each 

 other, the mesogloeal lamella, separating the meshes from the ectoderm, is namely often very thin (compare 

 the figure of U. felina = crassicornis O. F. Mxill. Mc. Murrich 1911 PI. 2, fig. 4). 



Also the distribution of the reproductive organs varies. I have stated 1893 that the 10 first pairs 

 of mesenteries of Urticina crassicornis (= U. felina coriacea) are sterile. I,ater on I have examined other 

 specimens and found this statement confirmed, or that also the 10 pairs of the second order are completely 

 or partly sterile. Mc. Murrich (1901 p. 34) declares that in U. crassicornis (a verrucous species from Puget 

 Sound) the two first cycles of mesenteries are devoid of reproductive organs. In two more closely examined 

 specimens fromHeUebsek (compare above), the size of which in contracted state was 0,7 cm in height and i cm 

 in breadth, but still provided with well-developed reproductive organs (testes with spermatozoa), it appeared 

 on sections that of the older mesenteries only 6 pairs were sterile. One specimen was provided with 40 pairs 

 of mesenteries and thus, as to the number of the mesenteries, in the stage, reproduced by Faurot (1895 

 p. 139). In the proximal part of the actinopharynx there were, however, 6 pairs perfect; whether in the distal 

 part some more mesenteries are perfect, I have not examined. Faurot has shown that the decamerism of 

 Urticina is due to the fact that the development of the mesenteries in the ventro-lateral compartments is 

 retarded. Thus in the dorso-lateral and the lateral compartments there is one cycle more than in the ventro- 

 lateral compartment. The 10 first pairs consist of 6 pairs of the first cycle and of 4 of the second (in the 

 dorso-lateral and lateral exocoels). The 10 following pairs, alternating with the former, are formed by 2 pairs 

 of the second order (in the ventro-lateral exocoels) and 8 pairs of the third order (in the other exocoels). The 

 20 following pairs have arisen as 4 pairs of the third order (ventro-laterally) and 16 pairs of the fourth order. 

 The arrangement of the reproductive organs of the specimens was as follows. The figures indicate the different 

 cycles, if we issue from a species with the mesenteries originally arranged after the number of 6. The decam- 

 erism of Urticina is namely, as above named, derived from a species with originally 6 pairs of mesenteries. 

 The spaced out figures indicate the fertile mesenteries, dm : directive pairs. 



dm dm 



14 3 424341434243413 2 313 2 31434243414342434 



The arrangement of the mesenteries in the second specimen, having 43 pair, was the following. 



dm d"" 



1 434524 3 41434243413234 132 3 41434243414342434 



In this specimen two pairs of the fourth cycle (in the ventrolateral compartments) and one pair of 

 the fifth (in a dorso-lateral compartment) are added. Here we find, that also the mesenteries of the third 

 cycle in the ventro-lateral compartments, and 3 pairs of the fourth cycle in a primary lateral compartment, 

 are provided with reproductive organs. 



If we compare those results with the former observations, we may conclude that the position of the 

 reproductive organs varies with the age of the animal; in the youngest specimen (an examined specimen still 



