COELENTERATES IN GENERAL 143 



type are free-swimming. Typically they have a bell-shaped body, with 

 a ring of tentacles around the margin of the bell, and a mouth, which also 

 may be surrounded with tentacles, in the center of the lower surface. 



Neglecting the many various modifications of both the polyp and 

 the jellyfish types, a typical polyp and a typical medusa may be directly 

 compared (Fig. 58). If one should imagine a polyp to be turned over 

 with the mouth directed downward; to be greatly broadened by lateral 

 extension of the body, which becomes bell-shaped; to have the tentacles 

 carried out to the margin of the bell; and then to show a great increase 

 in the amount of mesoglea, one would have an animal with some of the 

 marked characteristics of a jellyfish. The digestive portion of the enteron 

 lies more or less in a projecting manubrium which hangs down like a 

 clapper in the bell and at the tip of which is the mouth. The increase 

 in the amount of mesoglea, which is almost all water, lessens the specific 

 gravity of the body so that it is very little more than that of water. 

 This enables the jellyfish to float easily. The increase in the amount 

 of mesoglea, however, renders necessary the development of a system 

 of canals to put the enteron in communication with all parts. This 

 need is met by radial canals leading from the central enteron outward 

 to a circular marginal canal, the latter in turn being in communication 

 with the canals of the tentacles. 



170. Classification. — The phylum Coelenterata (sel en ter a' ta; G., 

 koilos, hollow, and enteron, intestine) is divided into three classes: 



1. Hydrozoa (hi dro zo' a; G., hydra, water serpent, and zoon, animal). 

 Includes fresh-water hydroids, colonial marine hydroids, floating hydroid 

 colonies like the Portuguese man-of-war, some of the smaller jellyfishes, 

 and the polyps which produce the stag-horn coral. 



2. Scyphozoa (si fo zo' a; G., skyphos, cup, and zoon, animal). — 

 Includes the larger jellyfishes. 



3. Anthozoa (an tho zo' a; G., anthos, flower, and zoon, animal). — 

 Contains the sea anemones, most of the coral-producing polyps, and also 

 those colonial forms known as sea fans and sea pens. 



171. Hydrozoa. — Among Hydrozoa the polyp type prevails. The 

 hydra is an example of this group. Though they may be variously 

 modified, hydrozoan polyps are always comparatively simple. The 

 hydroid colonies have a pronounced superficial resemblance to plants, 

 which they were at one time supposed to be. This led to the name of 

 zoophytes — literally, animal plants — now rarely used. These colonies 

 are found attached to various objects in the water, sometimes completely 

 hiding the surface of the object and extending outward to a distance of 

 several inches (Fig. 59). The jellyfishes belonging to this class are 

 characterized by the possession of a velum, a circular shelflike fold which 

 runs inward from the margin of the bell and incloses a chamber below 

 the body (Fig. 66). The velum assists in locomotion by alternate 



