CCELENTERATA. 21 



functions alimentation and reproduction; in roost cases, at a 

 certain stage, the reproductive element separates, and thenceforth 

 leads an independent existence. The alimentary element is 

 termed the "polypite," and is either single or, more frequently, 

 there is a large number united by the "coenosarc," which is usually 

 invested by a chitinous covering called the "polypary." In 

 Sertularida the polypary is composed of little cells or calycles 

 [hydro thecas], in each of which an individual is lodged. This is 

 the product of continuous budding. 



One of the forms of non-sexual reproduction is the "zooid." 

 Zooids differ from organs in that the zooid is an individual 

 organism, which may or may not be capable of independent 

 existence. A community of zooids in union with one another 

 constitutes the " hydrosome." Zooids are of two kinds: in one, 

 destined for the nutrition of the community, the assemblage is 

 called the "trophosome;" the other gives origin to the generative 

 elements ova and spermatozoa ; and the entire association of 

 these generative zooids is called the " gonosome." The tro- 

 phosome is composed of the "hydranth" and the " hydrophyton." 

 The hydranth (or polypite) contains the digestive sac ; the hydro- 

 phyton (or coenosarc) is, as we have seen, the common basis by 

 which the general community is kept together. The hydrorhiza 

 is the adherent base. The ultimate zooid, which generates either 

 the ova or the spermatozoa, is the " gonophore." The " sporo- 

 sac" is a gonophore without the umbrella. The "gonozoid" is 

 the sexual zooid, whether fixed or detached and fitted for loco- 

 motive life; this is also known as a "medusiform gonophore," or 

 " planoblast." The "gonangium," or "gonotheca," is an external 

 receptacle in which the gonophore is formed. 



In the development from the egg the embryo in the ciliated and 

 locomotive stage is known as a "planula." It is, however, not 

 flat, as the name would imply, but conical or cylindrical. In a 

 short time it loses its cilia and, with them, the power of active 

 locomotion, and is gradually changed until it acquires its adult 

 form. The planula consists of ectoderm and endoderm ; and the 

 primitive digestive cavity is formed by the invagination of the 

 ectoderm ; in Hydra, however, the mouth is produced directly 

 from the body-wall. In Tubularia, and probably some others, 

 the embryo assumes a radiate appearance, and is then known- as 

 an " actinula." 



For Glaus Hydroida is an order with four suborders Tabu- 

 lata, Tubularias, Carnpanulariae, and Trachy medusas. For Von 

 Hayek it is a class with six orders Hydrida, Corynida, Cam- 

 panularida, Sertularida, ^Eginida, and Graptolithida. 



