l86 ZOOLOGY 



attachment with the parent stock and, in the perfect medusae, 

 becomes free-swimming. 



229. Classification. — The following classes of Coelenterata may be recognized. 

 Class I. Hydrozoa (Hydra-like animals). — Hydrozoa are codenterates with two 



cell-layers (ectoderm and entoderm), between which there is a supporting layer 

 (the mesoglosa) non-cellular in structure. The reproductive cells arise chiefly 

 from the ectoderm. The life cycle may consist of polyps alone {Hydra); or of 

 medusae alone; or of both in one life history (Campanularia, Pennaria, Obelia). 

 Medusoid forms may be free or attached. The gastro-vascular cavity is not 

 divided by mesenteries. Here are included all the rather scarce fresh-water 

 coelenterates, many tubular marine forms somewhat similar to Hydra, and the 

 much diversified colonies of the Siphonophora (as the Portuguese Man-of-War, 

 found in mid-ocean, especially in the region of the Gulf Stream). See Pigs. 86, 87. 



Class II. Scyphozoa (cup animals). — Coelenterates in which the mesenchyma 

 contains cellular elements. The reproductive cells arise from the entoderm and 

 escape into the digestive cavity. Chiefly medusoid forms, though in some the 

 bell-form alternates with a polyp stage. Types: Aurelia and the larger jelly- 

 fishes. The majority of the Scyphozoa swim on the surface of the ocean; some are 

 found at considerable depths. Many of them are very large and handsome. An 

 especially interesting fact in connection with the development of such a type as 

 Aurelia is that its polyp (known as the Scyphisioma) is intermediate in its charac- 

 teristics between the polyps of the Hydrozoa and those of the Actinozoa. The 

 Scyphistoma has four ridges which partly separate the gastro-vascular cavity as do 

 the mesenteries in the Actinozoa. 



Class III. Actinozoa (ray animals). — Coelenterates with only the polyp form. 

 Cells in the mesenchyma. There is a well-developed ectodermic gullet (stomo- 

 daeum). The gastro-vascular cavity is more or less completely divided into 

 chambers by mesenteries. Sexual cells entodermal. A skeleton of calcareous or 

 horny material often present. 



Types: Sea-anemones; sea-fans and corals. The sea-anemones or sea-roses 

 are common on rocks and other objects just below low-water mark. Though 

 attached, they have some power of gradually changing their position. Species 

 of sea-anemones are known in which the individuals are as much as two feet in 

 diameter, though polyps of the colonial forms are usually very small. 



Class IV. Ctenophora ("comb-bearers"). — The Ctenophora are free-swimming, 

 pear-shaped jelly-fishes, never occurring in colonies, and not associated with a 

 polyp stage. They bear eight meridional plates supplied with transverse rows of 

 cilia, which function as locomotor and possibly as respiratory organs, and suggest 

 the name of the group. There is a well-developed stomodaeum. The gastro- 

 vascular canal branches from this and is much divided, one division lying under 

 each row of combs. There are two small aboral openings to the digestive canai , 

 known as excretory pores. The mesenchyma is well developed. There are no 

 nettling cells as in the other coelenterates; but glue-cells with similar structure and 

 functions are found. (Some students of the Ctenophora place them in a separate 

 phylum.) 



230. Ecology, etc. — The food of Coelenterates consists 

 largely of organic debris broken up by the waves, and of small 



