Phylum II. COELENTERATA. 



CoELENTERATES 01" Zoophytes are free-swimming or attached aquatic animals 

 of very variable form and size. They diflfer from the Protozoa in having 

 multicellular bodies with distinct organs ; and from all higher classes in the 

 absence of a definite body-cavity. In the subphylum Porifera there is a 

 simple or usually complex system of digestive sacs, with inhalent pores in the 

 body wall and one or many exhalent pores or oscula, and no stinging cells or 

 tentacles. The two other subphyla, Cnidaria and Ctenophora, exhibit a more 

 or less pronounced radial symmetry, have no inhalent pores and no special 

 exhalent opening in the body wall, but a large mouth opening conducts into a 

 gastrovascular cavity. Food is taken in and the excreta and sexual elements 

 are voided through the mouth opening. Stinging. cells and usually tentacles 

 are present in the two last-named divisions. 



The body consists of two layers of cells — an ectoderm and entoderm — 

 and usually also a third layer, the mesoderm. The ectoderm in the Cnidaria 

 often secretes a calcareous or horny skeleton, but in the Porifera the horny, 

 siliceous or calcareous skeletal elements are the product of the mesoderm. 



Reproduction is either sexual or asexual, or, in the Hydrozoa, an alterna- 

 tion of generations may occur. The process of budding or self-division gives 

 rise to polyzooid colonies, in which the zooids 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 subphyla : Porifera, Cnidaria and Ctenophora. Of these only the first 

 two have skeletons and have left traces in the rocks. 



Subphylum I. Porifera Hogg. 



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 single layer of collared epithelial cells constituting the entoderm, 

 and usually a strongly developed mesoderm, 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 siliceous 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 

 of water ladeu with food-particles. The pores communicate by means of fine 



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