142 MANUAL OF ZOOLOGY. 



3. PennatulidcR. Sclerobasis sulcate, free; soft parts in mul- 

 tiples of four. 



4. Gorgonidce. Sclerobasis sulcate, attached proximally ; 

 soft parts in multiples of four. 



Sclerodermic coralla fall under two heads, according as they 

 are simply composed of scattered spicules, or are provided with 

 true thecce. 



I. Spicular coralla occur in the Zoantharia Malacodermata 

 (occasionally), and in the Alcyonida ; and no differences can 

 be stated between the coralla themselves. The animals, how- 

 ever, differ entirely, the soft parts of the former being in mul- 

 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 Actinozoa: i. In \h& Zoantharia Sclerodermata ; 

 2. In the Tvbiporidce, amongst the Alcyonaria; and, 3. In 

 the Rugosa ; and the following are the distinctions between 

 them : 



1. Zoantharia Sclerodermata. Septa in multiples of five or 

 six, sometimes absent ; tabulae often present. 



2. Tubiporida. Septa absent; thecae united externally by 

 distinct, horizontal " epithecae." 



3. Rugosa. Septa in multiples of four; tabulae usually 

 present. 



CHAPTER XVI. 



CTENOPHORA. 



ORDER IV. CTENOPHORA. The Ctenophora comprise " trans- 

 parent, oceanic, gelatinous Actinozoa, swimming by means of 

 ' ctenophores] or parallel rows of cilia 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. 52) may be taken as the type of 

 the order, the structure of all being similar to this in essential 

 points. Pleurobrachia possesses a transparent, colourless, 

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



