100 COMPAKATIVE ANATOMY. 



in others, eight (Octactinia). In tlie first two divisions this number 

 is not permanent^ but the tentacles are soon increased in number, 

 and there is a corresponding change in the internal organisation. 

 A larger number of transverse axes can then be made out in the 

 organism, but their fundamental number in most cases remains 

 the same as at first. In the Octactinia, however, the first four 

 transverse axes persist. 



The body of the young animal is generally cylindi'ical, but this 

 form is retained in a few divisions only (Cereanthus, Actinia). In 

 the other forms colonies are built up, and this causes the external 

 appearance to vary greatly. The stocks (Polypariae) are formed 

 either by incomplete division, or by gemmation, or by both com- 

 bined. 



Longitudinal division aids in the formation of the colony to a 

 varying extent. In many cases it is merely indicated by transverse 

 growth, and does not lead to any division of the organism, as in 

 many Fungise. In others, division aifects the oral surface only, and 

 the internal parts remain continuous. When this process goes on 

 for some time, colonies with a large number of orifices are formed, 

 which are arranged in variously-curved rows, beset at their edges 

 with tentacles (Mseandrina). Whilst flattened or extended racemose 

 colonies are formed in this way, branched stocks are formed when 

 division is combined with a considerable growth of the persons in 

 a longitudinal direction ; and these stocks may not only vary in size 

 but also be branched in various ways. In the same way gemma- 

 tion may be the cause of the formation of complicated colonies. In 

 either case there is a portion of the body (coenosarc, coenenchyma) 

 common to all the persons, and belonging to the common stock. 

 The basal portion of the stocks of those OctactiniEe, which are not 

 fixed but set loosely in the mud or sand, are developed from this 

 coenosarc and form a solid stalk-like portion of the stock, in which 

 gemmation does not occur (Pennatulidas). 



§ 78. 



In the Ctenophora, which is the division differing most from the 

 other Acalephte, the permanent form of the body is developed from 

 a larva, which in all essential points is similar to that of the others. 

 In the Ctenophora there are four secondary axes, jDcrpendicular 

 to the primary, and the most important organs are arranged 

 conformably to these. The body therefore follows, generally, the 

 radiate type, which is best developed in the Beroidse. This eight- 

 rayed form is, however, derived from a four-rayed form, each radius 

 having been split up into two. Each of the two radii which arise 

 from a primitive radius are equivalent to the opposite radii of the 

 same transverse axes. The development of the body follows the 

 poles of one of the two transverse axes. The differentiation which 

 arises in this way is clearly seen in the Cydippidte; it is more 



