THE ANTHOZOA 



but it is much less convenient for practical purposes than the 

 term zooid, which is applied to the individuals which compose 

 colonial organisms in several other groups in the animal kingdom. 

 There is no inconvenience in applying the same general term to 

 the individual members of diflerent groups, if it is clearly under- 

 stood at the outset that there are several kinds of zooids, differing 

 from one another in important anatomical features, and if we bear 

 in mind that the term is more particularly applicable to the 

 asexually produced individuals composing a colony, but may 

 also be transferred to individuals, similar to the colonial forms in 

 all respects, except that they do not form colonies. Thro\;ghout 

 this chapter, the term zooid will be employed instead of the older 

 term polyp, to designate an Anthozoan individual. It is true 

 that Kcilliker has used the term, in a special and limited sense, in 

 describing certain Anthozoa, but his special use of the term is 

 unwarrantable, and will be referred to further on. 



A Coelenterate zooid is an animal consisting of a hollow sac 

 of various form — columnar, spherical, or disc shaped. The cavity 

 of the sac, known as the coelenteron, is the only cavity of the 

 body, and communicates with the exterior by an opening, the 

 mouth, which serves the double purpose of admitting food into 

 the cavity of the sac, and of expelling undigested matter ; and in 

 the Anthozoa the reproductive elements. There is rarely a second 

 aperture at the end of the body furthest from the mouth opening. 

 A vertical line passing through the centre of the mouth is the 

 principal axis of the coelenterate body, the secondary axes being 

 disposed radially with regard to the principal axis, though, as will 

 be seen further on, there ai-e many cases in which the primitive 

 radial symmetry is replaced by a more or less well-defined, 

 bilateral symmetry. Around the mouth, but placed at some little 

 distance from it, is a circlet of tentacles disposed radially with 

 regard to the principal axis. The space between the mouth and 

 tentacles is known as the peristome. The tentacles may be 

 solid or hollow ; when hollow, their cavities are prolongations of 

 the coelenteron. 



The walls of the sac-like body, and also the tentacles and 

 peristome are always composed of three layers of tissue, of which 

 two, the external layer or ectoderm, and the internal layer or 

 endoderm, are always cellular, and are coextensive and identical 

 with the epiblast and hypoblast of the embryo. 



The third layer, lying between the ectoderm and endoderm, 

 varies considerably in structure and importance in different groujis 

 of the Coelentera. Typically, it is not a cellular layer, but is of 

 gelatinous consistency, and is formed as a sort of secretion from 

 the ectoderm ; in some cases the endoderm also takes a share in 

 its formation. After treatment with reagents, the middle layer 



