No. i.] MORPHOLOGY OF THE ACTINOZOA. 1 43 



There are some points in the histology of the younger larvae 

 which deserve mention. The ectoderm is comparatively high, 

 and lacks as yet all trace of a definite cuticle. Its cells present 

 the characteristic Hexactinian specialization, there being large 

 numbers of nematocysts and gland-cells present, the latter being 

 of the two varieties described by the Hertwigs ('79). In the 

 upper portion of the embryo the granular cells are less numer- 

 ous than towards the aboral end, where they are exceedingly 

 abundant. Immediately external to the mesoglcea is a thin 

 layer of coarsely granular substance, in which no cells can be 

 distinguished ; it has the position occupied by the longitudinal 

 muscles of Cerianthus, but I cannot obtain any evidence of 

 muscle fibres in it. External to it is a clear band, which seems 

 to be a layer of nerve fibres. The cells which bear the long 

 cilia forming the ciliary ring are markedly different from those 

 composing the general ectoderm, being considerably lower, and 

 lacking both gland-cells and nematocysts. Apparently, no aboral 

 tuft of cilia is present. 



A second peculiar feature is the presence of two ectodermal 

 papillae close to the mouth (Fig. 5, e. p). They are not very high, 

 but still are well' marked ; the mesoglcea takes no part in their 

 formation, but they are entirely ectodermal structures, resem- 

 bling that layer elsewhere on the body in their histological 

 details, but being characterized by a special development of 

 gland-cells. Perhaps they represent the dentations which occur 

 upon the margin of some forms of Zoanthus. 



The mesoglcea of Zoantheae, as is well known, is character- 

 ized by being traversed by canals containing cells, and by elon- 

 gated fibril-like cells, in this respect differing from the meso- 

 gloea of other Actinozoa, the Alcyonaria excepted. The origin 

 of these cells is of some interest. They were discovered by 

 Kolliker (66), who observed that prolongations passed outwards 

 from the canals to join the ectoderm, and the same fact has been 

 recorded by R. Hertwig ('82), Erdmann ('85), and myself ('89), 

 though I also obtained evidence that the endoderm in adult indi- 

 viduals also contributed to the formation of the mesoglceal en- 

 closures. Near the base of each mesentery there is also a large 

 canal imbedded in the mesoglcea, and the contents of this Hert- 

 wig is inclined to consider endodermal in origin, basing his 

 opinion mainly upon the fact that he found Zooxanthellae in 



