No. i.] MORPHOLOGY OF THE ACT I NO ZO A. 145 



the endoderm has disappeared. This dissolution of the bound- 

 ing wall takes place where it is thinnest, the mesoglcea being 

 always thinner on one wall than on the other, the thick wall 

 being that upon which the longitudinal muscles will be devel- 

 oped. 



The disintegration of the bounding wall must later be repaired, 

 since in the older specimen the separation between the canal 

 and the endoderm is perfect, and both walls are equal in thick- 

 ness. Furthermore, the canals have extended up into the 

 stomatodaeal region, and their section there is quite as large as 

 it is lower down. The canals still retain the same relative dis- 

 tances from the base of the mesentery which they possessed 

 previously, the increase in length of the imperfect mesenteries 

 having been produced by the formation of new mesoglcea dis- 

 tally to the canal, a fact that would seem to indicate that the 

 endoderm takes part in the formation of the mesoglcea. The 

 smaller canals, which in the younger larvae occurred at the 

 bases of the mesenteries, are in the older specimens much 

 larger, nearly equalling in size the mesenterial canal. The con- 

 tents of the canals still present the same histological character 

 as in the earlier stage. 



Van Beneden, in his paper containing a description of Semper's 

 larva, discusses at some length the affinities of the various tribes 

 of Actinians. He lays great stress upon the imperfection of 

 six of the twelve mesenteries that are developed in the Zoanthid 

 larvae, and believes that there can be no doubt but that "il 

 existe dans le cours de revolution de notre larve un stade long- 

 temps prolonge pendant lequel l'organisme se caracterise par la 

 presence de six sarcoseptes primaires." I cannot see that the 

 fact that there are only six perfect mesenteries justifies this 

 conclusion in the least. It would have been quite as reasonable 

 before these larval forms had been studied, to conclude from 

 the study of adult Zoanthids with, let us say, twenty pairs of 

 mesenteries, that in the course of their evolution they passed 

 through a stage, persisting for a considerable time, in which 

 they were characterized by the presence of twenty primary sar- 

 cosepta. Or, to take another case, if the larvae of the macro- 

 typal Zoanthids be found to possess in the twelve mesenteried 

 stage eight perfect mesenteries (which seems quite probable), 

 one would be justified in concluding, on the same grounds that 



