SECT, IV 



PHYLUM CCELENTERATA 



2] 5 



with the stomodfeum, 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 hase (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 \^'hich the combs 

 arise being particularly large. The epithelium of the stomoda3um 

 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- 



cxi.c 



Lid I 









'^iMim 





)-^uc 



_^,A.^_ 



-^, 



=A,,, 



Fio. liil. — Hormiphora plumosa. A, transverse section of one of the branches of a 

 tentacle ; B, two adhesive cells {ad. c) and a sensory cell (s. c.) highly magnified, cw. cuticle 

 nw. nucleus. (After Hertwig 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 



