PHYLUM CCELENTERATA 209 



3. GENERAL ORGANISATION. 



Compared with the two former classes of Coelenterates, the Hydrozoa and 

 Actinozoa, the organisation of the Ctenophora is remarkably uniform. This is 

 due to the fact that all the species are pelagic, none are colonial, and none form 

 skeletons. Nevertheless a very great diversity of form is produced in virtue of 

 differences in proportion and modifications of the tentacular and canal systems. 



The Cydippida agree in all essential respects with Hormiphora, the most 

 important deviation from the type-form being the compression of the body in the 

 transverse plane in some genera, e.g. Euchlora (Fig. 155, 2), the result being an 

 oval instead of a circular transverse section, with the tentacles at the end of the 



mm 



l.Callianira 



2.Euchlora 



S.LamjaeHa 



Fir;. 1 :>.>. Three Cydippida. ah. i>. aboral process ; ;nth. mouth. (After I'iiui. ) 



long axis. The aboral pole may be produced into wing-like appendages, as in 

 CaUifinira (1), and in Lampetia (3) the mouth is so dilatable as to form, when 

 expanded, a sole-like plate by which the animal retains itself on the surface of the 

 water or creeps over submarine objects. In Euchlora rubra minute nematocysts 

 have been found, and there is reason to believe that it was by the modification 

 of these characteristic coelenterate organs of offence that the adhesive cells of 

 Ctenophora were evolved. 



The Lobata, for instance Deiopea, are distinguished, as their name implies, 

 by the presence of a pair of large lappets (Fig. 156, lp.) t into which the oral 

 surface is produced at either end of the vertical plane. Four of the swimming 



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