ACTINOZOA. ' 201 



equal in number and opposite to those of the 

 outer. 



The tentacles of most Zoantharia.siTe retractile, 

 but in CerianthuSf Anthea, and a few other 

 forms, this power is either absent, or imperfectly 

 exercised. 



The numerous families of the present order 

 have been conveniently arranged by Milne Ed- 

 wards under three sub-orders : Malacodei^iata ; 

 Sclerobasica ; and Sderodermata. 



In the Z. Malacodeimata, the corallum is 

 either absent or represented by scattered spicules. 

 The actinosoma, in most of these animals, presents 

 but a single polype. Exceptions to this rule oc- 

 cur, however, among the ZoantJiidce, the budded 

 polypes of which remain permanently united by a 

 coenosarc, in some linear, in others carpet-like or 

 incrusting. In certain Actinidce also, for example, 

 the Corynadis mediterranea of Sars, a similar 

 connection is maintained {fig. 34, d). 



Within the soft parts of the Z. sderobasica 

 spicular tissue secretions seem wanting (fig, 35). 

 All the members of this sub-order are composite 

 structures. Antipatkes, the type of the group, 

 presents a stem-like, simple or branched coenosarc, 

 which in one species tapers to a length of more 

 than nine feet, with a basal diameter of scarcely 

 •3 of an inch. In this genus the sclerobasis is 

 horny, and each polype, according to Dana, has 

 but six tentacles ; but in the allied family of 

 Hyalodioetidce, the tentacles are twenty in num- 

 ber, while the basal excretion resolves itself into 

 numerous siliceous threads, transparent, twisted 

 into an erect axis. Doubts, however, are yet 

 entertained of the true nature of these so-called 



