Two Peculiar Actinian Larvcc from Tortugas, Florida. 



■S5 



MESOGLCEA. 



The mesogloea is generally thicker in Zoanthella than in Zoanthina and 

 varies considerably in thickness in different parts of the same larva. In both 

 types of larvae it is thickest in the aboral portion of the body and thinnest 

 beneath the ciliated band. The extreme thinness of the supporting lamella 

 in the region of the ciliated band as contrasted with its thickness else- 

 where is well shown in text-figure 3 and in plates 2 and 3. The same 

 figures show that in Zoanthella this layer is thinner opposite the point of 

 origin of septa than in the region between septa, while in Zoanthina the re- 

 verse is true. Cells and canals are found within the mesogloea in both larvae, 

 though they are more abundant in Zoanthella. There is considerable evi- 



FlG. 5. — Cross-section through body-wall of Zoanthina, showing 

 mesogloea and endoderm. X 333- 



dence in favor of the view of Van Beneden that the mesogloea is in life quite 

 soft and that the cells found in it are wandering cells and the canals merely 

 the tracks of these cells. 



In my oldest specimen of Zoanthina the mesoglcea in each of the macro- 

 septa is drawn out into many fine branches on the thicker side of the sep- 

 tum, thus giving support to the longitudinal muscles of the septa. In Zoan- 

 thella the flexure of the body toward the ventral side gives evidence of the 

 presence of strong longitudinal muscles along that side, though I have not 

 been able to distinguish them in my sections. Van Beneden has seen and 

 described these muscles in the specimens which he studied. 



In none of the specimens which I have examined were there any em- 

 bryos within the coelenteron, such as Van Beneden discovered in Zoanthina 

 nationalis. These embryos he shrewdly concludes were merely ingested by 

 an older embryo, while all were contained within a coelenteron of the vivi- 

 parous parent. If this is the true explanation of their presence in Van 

 Beneden's specimen, it may be concluded that a more extensive study of the 



