iv PHYLUM CCELENTERATA 183 



branches. The stem is supported by a calcareous or horny 

 skeleton. The polypes are dimorphic. The " Sea-pens" (P< // natula ; 

 are the commonest members of this group (Fig. 134). 



Sysfrnt<'<fit' 7'uxifion of the Example. 



Tealia crassicornis is one of several species of the genus 

 Tealia: it belongs to the family TcaUdce, which, with several 

 other families, make up the tribe Hexactinicv, of the order 

 Actiniaria, of the sub-class Zqantharia. 



The presence of numerous tentacles, arranged in multiples of 

 five, places it at once among the Zoantharia. The fact that it is 

 simple and devoid of a skeleton causes it to be assigned to the 

 Actiniaria. This order is divided into tribes characterised by 

 differences in the arrangement of the mesenteries, especially by 

 the presence of one or two couples of directive mesenteries, and 

 by the direction in which the longitudinal muscles are turned. 

 In the Hexactinise the mesenteries are all arranged in couples 

 with the longitudinal muscles of each turned towards one another, 

 except in the case of the two directive couples. The mesenteries 

 are in multiples of five, and the stomodasimi has two siphonoglypb.es 

 and two lappets. 



The family TealidaB is characterised by the possession of 

 numerous mesenteries, tentacles of moderate length, which are 

 completely covered by the closed-in disc during retraction, and 

 by the presence of a large endodermal sphincter muscle. The 

 genus Tealia is distinguished from other members of the same 

 family by being broader than high, by having numerous retractile, 

 equal-sized tentacles, and by the presence of longitudinal series 

 of warts on the column. The species crassicornis is distinguished 

 from other species of the genus by the warts being of approxi- 

 mately equal size. 



3 GENERAL ORGANISATION. 



The chief variations in the external form of the Actinozoa are 

 due to the diverse modes of budding : as we shall see, the structure 

 of the individual polypes or zooids is remarkably uniform at 

 least as regards all the essentials of their organisation. 



Nearly all the Actiniaria or Sea-anemones are simple, and, in the 

 few instances where colonies are formed, these are usually small, 

 and contain a very limited number of zooids. In Zoriiitkii* 

 (Fig. 130), for instance, the original polype sends out a horizontal 

 branch or stolon (st.), from which new polypes arise. Besides the 

 Sea-anemones the only simple forms are certain Madreporarian 

 corals, such as MaMlvm (Fig. 142, A, B), and three genera of 

 Alcyonacea, of which Hartca (Fig. 131) may be taken as an 

 example. 



