sect, iv PHYLUM CCELENTERATA 91 



Class 2. — Scyphozoa, including most of the large jelly- 

 fishes. 



Class j. — Actinozoa, including the sea-anemones, and 

 the vast majority of stony corals. 



Class 4. — Ctenophora, including certain peculiar jelly- 

 fishes known as "comb-jellies." 



1. THE HYDROZOA 



Obelia, which is a good example of the class, is a common 

 zoophyte occurring in the form of a delicate, whitish, or light 

 brown, almost fur-like growth on the wooden piles of piers 

 and wharfs. Obelia commissuralis occurs on the coast of 

 New England almost at low-water mark, being exposed only 

 at the lowest tides. With it, north of Cape Cod, may be 

 found Obelia gelatinosa, a rather stouter species, but similar 

 in general appearance. Obelia geniculata is abundant on 

 Laminaria or the "devil's apron," giving the fronds when 

 submerged a downy appearance. The following account 

 refers to a common European species : It consists of 

 branched filaments about the thickness of fine sewing 

 cotton ; of these, some are closely adherent to the timber, 

 and serve for attachment, while others are given off at right 

 angles, and present at intervals short lateral branches, each 

 terminating in a bud-like enlargement. The structure is best 

 seen under a low power of the microscope. The organism 

 (Fig. 41) is a colony, consisting of a common stem or axis, 

 on which are borne numerous zooids. 



The large majority of the zooids have the form of little 

 conical structures {P, i-P, 4), each enclosed in a glassy, 

 cup-like investment or hydrotheca {htK), and produced dis- 

 tally into about two dozen arms or tentacles (t) : these 

 zooids are the polypes or hydranths. Less numerous, and 



