PHYLUM CQ5LENTKRATA 



215 



with the stomodreum, and a tentacular canal (t. c.) which ex- 

 tends outwards and downwards into the base of the correspond- 

 ing tentacle. Each tentacle presents a thickened base (t. b.), 

 closely attached to the wall of the sheath, and giving off a long 

 flexible filament, beset with processes of two kinds — one simple 

 and colourless, the other leaf-like, beset with branchlets, and of a 

 yellow colour. 



Cell-layers. — The body is covered externally by a delicate 

 ectodermal epithelium (Fig. 160), the cells from which the combs 

 arise being particularly large. The epithelium of the stomodoeum 

 is found by development to be ectodermal, that of the infundibulum 

 and its canals endodermal : both are ciliated. The interval between 

 the external ectoderm and the canal-system is filled by a soft jelly- 

 like mesogloea. The tentacle-sheath is an invagination of the ecto- 



a<i.c 



PlO. 161, Hormiphora plumosa. A, transverse section of one of the branches of a 

 teataeb ; B, two adhesive cells (ad. c.) and a sensory cell (s. c.) highly magnified, cm. cuticle 

 nu. nucleus. (After Uertwig and Chun.) 



derm, and the tentacle itself is covered by a layer of ectoderm, 

 within which is a core or axis formed by a strong bundle of longi- 

 tudinal muscular fibres, which, as we shall see, are of mesodermal 

 origin, and which serve to retract the tentacle into its sheath. 



Delicate muscle-fibres lie beneath the external epithelium and 

 beneath the epithelium of the canal-system, and also traverse 

 the mesogloea in various directions. The feeble development 

 of the muscular system is, of course, correlated with the fact 

 that the swimming-plates are the main organs of progression, 

 the Ctenophora differing from all other Ccelenterata in retaining 

 cilia as locomotory organs throughout life. 



A further striking difference between our present type and the 

 Ccelenterata previously studied is the absence, in Hormiphora, of 

 stinging-capsules. The place of these structures is taken by the 

 peculiar adhesive-cells with which the branches of the tentacles 



