46 ZOOPHYTES. 



We glance at some of the principal varieties of structure in each of 

 these divisions. 



I. AcTiNARiA. 40. In one of the divisions of this group — the Anti- 

 pathacea — the polyps have but six tentacles.* In the tentacles and 

 the general appearance and habit of the polyp, they resemble the 

 Madrepores, but no dissection has yet been made of thera. 



41. The polyps of the Madreporacea, another division of the Acti- 

 naria, are distinguished by twelve tentacles in a single series. f There 

 is among them but little variety of structure. In a few species the 

 alternate tentacles are of unequal size, and it is probable that the vis- 

 ceral lamellae have the same inequality : and in some Madrepores one 

 of the tentacles is elongated and more flexible than the others :% again, 

 all are sometimes obsolete. 



The character of the cells formed by Madrepores show that in 

 many of them two opposite interlameJlar spaces in the visceral cavity 

 are generally broader than the others, as in the Actiniae, and usually 

 these two are very unequal. 



Tlie only fleshy species known, which has the twelve tentacles of 

 the Madrepores, is the Actinia davus of Quoy and Gaymard,^ found 

 by them entangled in the tentacles of a Medusa, off" the coast of New 

 Holland. By their dissections, the stomach and visceral laraelte are 

 shown to have nearly the characters of those in the Palythoa, except 

 that tiie lamellae are equal and are twelve in number, corresponding 

 to the tentacles. The appearance of this Actinia expanded is very 

 similar to that of a madrepore polyp. 



The most marked variation from the usual character of the Ma- 

 dreporacea is found in the Porites family. In these the visceral 

 cavity does not extend to the base of the animal, as in the otlier species 

 of the group, and the visceral lamellae cannot be traced through the 

 tissues of this portion. This structure is indicated by the porous coral 

 secretions formed by this part of the polyp. In external characters 

 these polyps present few peculiarities. A few allied species (Gonio- 

 porae) appear to have more than twelve tentacles, though all are in a 

 single series, as in others of the Madreporacea. 



42. The polyps of the Caryophyllia and Ast?-cca tribes have more 

 than twelve tentacles, and they are in two or more series or scattered. 



* Plate 56, figures 1 and 2. ■\ Plate 31, figure 1. J Plate 33, figure 1. 



§ Voyage de PAstrolabe, iv. 150, pi. 10, figs. 6 to 11. It is yet doubtful whether 

 this species was actually an adult with its full number of tentacles. Several individuals 

 were seen, which were about three-fourths of an inch long when expanded. 



