212 



ZOOLOGY 



SECT. 



three tubes are given off : one, the infundibular canal (inf. c.), passes 

 directly upwards, and immediately beneath the aboral pole divides 

 into four short branches, two of which open on the exterior by 

 minute apertures, the excretory pores (Fig. 161, A, ex. p.). The two 

 other canals given off from the infundibulum are the per-radial 

 canals (per. c.) : they pass directly outwards, in the transverse plane, 

 and each divides into two inter-radial canals (int. c.}, which in their 

 turn divide each into two adradial canals (adr. c.). These succes- 

 sive bifurcations of the canal-system all take place in a horizontal 

 plane (Fig. 161, B), and each of the ultimate branches or adradial 

 canals opens into a meridional canal (mrd. c.), which extends up- 

 wards and downwards beneath the corresponding swimming-plate. 

 Furthermore, each per-radial canal gives off a stomodceal canal 

 (std. c.), which passes downwards, parallel to and in close contact 

 with the stomodseum, and a tentacular canal (t. c.), which extends 



ad.c 



102. Hormiphora plumosa. A, transverse section of one of the brandies of a 

 fentacle ; B, two adhesive cells (ad. c.) and a sensory cell (s. c.) highly magnified, cu. 

 cuticle ; MM. nucleus. (After Hertwig and Chun.) 



outwards and downwards into the base of the corresponding 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. 161), the cells from which the combs 

 arise being particularly large. The epithelium of the stomodseum 

 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 imagination of the ecto- 

 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- 



