124 MANUAL OF ZOOLOGY. 



4. Gorgonidce. Sclerobasis silicate, 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 theccz. 



I. Spicular coralla occur in the Zoantharia Malacodermata 

 (occasionally), and in the Alcyonidce; 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 the Zoantharia Sclerodermata. 

 2. In the Tubiporida, 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 pre- 

 sent. 



CHAPTER XVI. 

 CTENOPHORA. 



ORDER IV. CTENOPHORA. The Ctenophora comprise " trans- 

 parent t oceanic, gelatinous Actinozoa, swimming by means of 

 1 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. 34) 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 



