CCELENTERATA. 1 77 



the daughters remain in association with the parent. The 

 medusoid individual, often of a very much simpler type than 

 that described above ( 218), may be produced in a similar 

 way from a bud. It usually breaks its attachment with the 

 parent stock and becomes free-swimming. 



224. Classification. The following classes of Ccelenterata may be 

 recognized. 



Class I. Hydrozoa. Hydrozoa are Coelenterates with two cell-layers 

 (ectoderm and entoderm), between which there is a supporting layer (the 

 mesoglcea) 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, Pen- 

 naria, 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 ccelenterates, many tubular marine forms some- 

 what similar to Hydra, and the much diversified colonies of the Siphon- 

 ophora (as the Portuguese Man-of-War, found in mid-ocean, especially 

 in the region of the Gulf Stream). See Figs. 84, 85. 



Class II. Scyphozoa. Coelenterates in which the mesenchyma con- 

 tains 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 sur- 

 face 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 Scyphistoma) is intermediate in its characteristics 

 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, Coelenterates with only the polyp form. Cells 

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

 modaeum). 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 indi- 

 viduals 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 rows of vibratile plates 

 composed of cilia, which function as locomotor and possibly as respiratory 

 '3 



