THE GTENOPEOBA. 123 



the primary axis is practically wanting, the secondary tentacles 

 arising directly from the bottom of 

 the tentacle-sheath. The ectoderm of 

 the secondary tentacles contains nu- 

 merous cells supposed to be sensory, 

 and also so-called adhesive cells, which 

 in this group replace the nematocysts. 

 They consist of a slender spirally- 

 coiled muscular fibre (Fig. 68, m) at- 

 tached at one extremity to the subja- 

 cent tissue and terminating at the 

 other on the under surface of a hemi- Fig. 68.— Adhesive Cells 

 spherical cap (c), whose surface is ^^o'" Tentacle of a 



T , n 1 • 1 e CtbnOPHORB (after Chun). 



covered by small spherical masses of ^ ^ ^^^^^.^^ ^^p 

 a sticky secretion. A small animal m = contractile stalk 

 coming into contact with these caps 



is held by the adhesive secretion, the muscle-fibre being suf- 

 ficiently elastic to yield to the struggles of the victim and to 

 bring it in contact with the general ectoderm by contracting 

 when its struggles cease. 



The reproductive organs lie in the outer walls of the 

 canals which lie beneath the meridional rows of plates, but 

 apparently are originally derived from the ectoderm. All the 

 Ctenophores are hermaphrodites, the ova being arranged on 

 one side of each canal and the spermatozoa on the other, in 

 such a manner that the adjacent sides of any two canals bear 

 the same kind of sexual cells. A peculiar phenomenon 

 termed Dissogony has been observed in certain forms, consist- 

 ing of the occurrence of two periods of sexual maturity in the 

 life-history of the individual, the reproductive organs ripening 

 first while it is still in a larval stage and again when it has 

 reached its adult form. 



The main bulk of the body of a Ctenophore is made up by 

 a gelatinous tissue intervening between the endoderm and 

 ectoderm and which may be termed the mesogloea, though it 

 is not improbable that its cellular elements are in great part 

 derived from embryonic ceils corresponding to the mesoderm- 

 cells of higher forms. It consists of a gelatinous matrix 

 through which are scattered branched cells and fibres. Some 



