Sub-Kingdom II. COELENTERATA. (Zoopnyres) 
COELENTERATES or Zoophytes are free-swimming or attached aquatic animals of 
very variable form and size, with multicellular bodies, and more or less distinctly 
radialin symmetry. A large-sized mouth-opening conducts into a central gastric 
cavity (gastrovascular space), which either ends blindly, or is provided with peri- 
pheral pouches, or a canal-system for the conveyance of food-particles. Since 
the chief function of the gastrovascular space, together with its diverticula, is 
that of digestion, although including also the reproductive organs, it corresponds, 
at least physiologically, to the stomach and intestines of higher animals. A 
definite anal opening is absent ; the excretions and sexual elements are voided 
through the mouth. 
The body consists of three layers of cells, an ectoderm, mesoderm, and 
entoderm. The ectoderm often secretes a calcareous or horny skeleton, but in 
most cases the horny, silicious, or calcareous skeletal elements are the product 
of the mesoderm. 
Reproduction is either sexual or asexual, or an alternation of generations 
may occur. The process of budding or self-division gives rise to polyzooid 
colonies, in which the individuals subsist in intimate relationships with one 
another, and sometimes institute a physiological division of labour. 
R. Leuckart was the first to recognise the Coelenterates as constituting a 
distinct structural type of animals, and separated them from the Echinoderms, 
with which the older systematists had associated them under the general term 
of Radiates or Actinozoa. The Coelenterates are divided into three principal 
groups or sub-branches: Porifera, Cnidaria, and Ctenophora ; of which only the 
first two have left traces in the rocks. 
Sus-Brancn I. Porifera. Hogs. 
The Porifera or Sponges are sessile, aquatic animals of extremely variable 
form. The body consists of a single layer of pavement-cells forming the 
ectoderm, a likewise single layer of collared epithelial cells constituting the 
entoderm, and a strongly developed mesoderm tissue, which latter comprises the 
bulk of the soft parts (including all the organs, muscles, sexual elements, and 
nerves), and almost invariably secretes a hard skeleton. The latter may consist 
of horny sponge-fibres, or of regularly disposed silicious or calcareous skeletal 
elements. The whole body is ramified by a canal-system, and the outer 
epithelial layer is perforated by countless, minute, dermal pores for the entrance 
