130 MANUAL OF ZOOLOGY. 



tiples of five or six, those of the latter being in multiples of 

 four. 



II. A thecal sclerodermic corallum occurs in three distinct 

 sections of Adinozoa: — i. In the Zoantharia Sckrodermata. 

 2. In .he TubiporidcB, amongst the Alcyonaria; and, 3. In 

 the Rugosa; and the following are the distinctions between 

 them : — 



1. Zoantharia Sderodermata. — Septa in multiples of five or 

 six, sometimes absent ; tabulae often present. 



2. Tubiporidm. — Septa absent; thecae united externally by 

 distinct, horizontal "epithecse." 



3. Rugosa. — Septa in multiples of four ; tabulae usually pre- 

 sent. 



CHAPTER XVI. 



CTENOPHORA. 



Order IV. Ctenophora. — The Ctenophora comprise " trans- 

 parent, oceanic, gelatinous Actinozoa, swimming by means of 

 ' ctenophores,' or parallel rows of alia disposed in comb -like 

 plates. No corallum." — (Greene.) 



The members of this order are all free-swimming organisms, 

 and they are placed by many amongst the Hydrozoa, from 

 which, however, they appear to be clearly separated by the 

 possession of a differentiated digestive sac, as well as by their 

 analogies with the Actinozoa, and their generally superior 

 degree of organisation. 



Pleurobrachia (Cydippe) (fig. 36) may be taken as the type 

 of the order, the structure of all being similar to this in essen- 

 tial points. Pleurobrachia possesses a transparent, colourless, 

 gelatinous, melon-shaped body, or " actinosoma," in which the 

 two poles of the sphere are termed respectively the " oral " and 

 " apical," and the rest of the body constitutes the " interpolar 

 region." At the oral pole is the transverse mouth, bounded 

 by lateral, slightly protuberant margins. " Eight meridional 

 bands, or ' ctenophores ' bearing the comb - like fringes, or 

 characteristic organs of locomotion, traverse at definite in- 

 tervals the interpolar region, which they divide into an equal' 

 number of lune-like lobes, termed the " actinomeres " ; but 

 this division of the body does not extend into the immediate 

 vicinity of the poles, before reaching which the ctenophores 

 grradually diminish in diameter, each terminating in a point." — 



