ACTINOZOA. 139 



a Hydrozoon, wholly consisting of ectoderm and 

 endoderm. 



The entire class is divided into four orders. In 

 the first of these, Zoantharia, represented by 

 Actinia and its immediate allies, the number of 

 mesenteries, tentacles, and other parts in connec- 

 tion therewith is, in general, some multiple of five 

 or six. In the three remaining orders some mul- 

 tiple of the number four appears to prevail. Thus 

 in the Alcyonaria there are eight somatic cham- 

 bers, eight mesenteries, and eight tentacles, not 

 simple, as in Actinia, but furnished with pinnate 

 margins {Jig. 26, a and h). The members of the 

 third order, Rugosa, known only through its fossil 

 representatives, seem to have possessed a structure 

 in some respects intermediate between that of the 

 two preceding sections. Lastly, the Ctenophora 

 are free-swimming, gelatinous animals, in physiog- 

 nomy widely different from the other Actinozoa, 

 though evidently akin to these in their leading 

 anatomical features. They are the most highly or- 

 ganized of Coelenterate animals, a common repre- 

 sentative of the order, Pleurohrachia, possessing 

 a complex nutritive apparatus, together with 

 well-defined organs for prehension, reproduction, 

 locomotion, and, in addition, unmistakeable indi- 

 cations of a nervous system {Jigs. 27 and 39). 



Like the members of the preceding class, many 

 of the Actinozoa multiply freely by gemmation, 

 complex plant-like individuals being thus formed 

 which consist of numerous zooids united by a 

 coenosarc {figs. 34, d and 35). In such instances, 

 each nutritive zooid, or that portion of the organism 

 which answers to the polj^Dite of a Hydrozoon, is 

 distinguished by the name of ^polype.' When 



