THE AOTINOZOA AND THE HYDROZOA. 117 



III. THE CCELENTERATA. 



The ACTINOZOA. I cannot distinguish more than two 

 groups of ordinal value in this class. 



1. The CTENOPHORA. Free swimming organisms, provided 

 with symmetrically disposed bands of large cilia which sub- 

 serve locomotion. At the end of the body opposite the mouth 

 the common cavity opens externally in some species by two 

 apertures between which a nervous ganglion and otolithic sac 

 may be discovered. The mesenteries are very thick, while the 

 intermesenteric spaces take the form of canals. The genera 

 Beroe, Cydippe, Cesium, &c., belong to this order. 



2. The CORALLIGENA. These animals are organized upon the 

 same plan as the Actinia described above, with variations in the 

 number and proportion of the parts, and in the forms of the 

 masses which are produced by the gemmation or fission of the 

 first-formed individual. Most of them give rise to a calcareous 

 skeleton, which may have the form of detached spicula, of a 

 solid axis, or of a theea or cup for each actiniform zooid, or of 

 some combination of these. 



The Aetinife, Gorgoniee, and coral-forming animals in general 

 constitute this order. 



The HYDROZOA. Three divisions of ordinal value may be 

 distinguished among the animals constituting this extensive 

 class. 



1. The HYDROPHORA. The alimentary zooid, or polype, is 

 provided with numerous tentacles, which are either set round 

 its mouth or scattered over its surface. When free-swimming 

 reproductive zooids are developed, the genitalia are borne by a 

 nectocalyx, or swimming-bell, the inner margin of the aperture 

 of which is produced into a muscular velum. Such zooids are 

 always formed by budding, and are commonly called craspedote, 

 or gymnophthalmous, Medusae. 



The immediate product of the growth of the embryo is a 

 fixed Hydroid, Tubularian or Sertularian Polype. 



2. The SIPHONOPHORA. The alimentary zooid never bears 

 numerous tentacles around its mouth or on its surface, but such 



