92 MANUAL OF ZOOLOGY SECT, iv 



Class 2. Scyphozoa, including most of the large Jelly- 

 fishes. 



Class 3. 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 com- 

 mon Zoophyte occurring in the form of a delicate, whitish 

 or light brown, almost fur-like growth on the wooden piles 

 of piers and wharfs. It consists of branched filaments 

 about the thickness of fine sewing-cotton : of these, some 

 are closely adherent to the timber, and serve for attach- 

 ment, 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. 40) 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 (h.th.\ and produced 

 distally into about two dozen arms or tentacles (t.} : these 

 zooids are the polypes or hydrant hs. Less numerous, and 

 found chiefly towards the proximal region of the colony, 

 are long cylindrical bodies mblastostyles (bls.\ each enclosed 

 in a transparent case, the gonotheca (g.th.\ and bearing 

 numerous small lateral offshoots, varying greatly in form 

 according to their stage of development, and known as 

 medusa-buds (m.bd.). By studying the development of 

 these structures , and by a comparison with other forms, it 



