82 



Introduction to Animal Morphology. 



Fine bristles (palpocils) project from the walls of some 

 of the surface interstitial cells (seen on the thread cell 

 in Fig. 9, B ; protoplastic processes of T. S. Wright). 

 The endoderm cells are large, unilaminar, vacuolated, 

 often flagellate, often with no distinct cell wall. The 

 thread-like processes of the ectoderm cells, united by 

 a copious intercellular substance, form an intermediate 

 layer of vertical fibres* (described by Kolliker). 

 Amoeboid cells wander through both layers. The 

 tentacles are not transversely septate, and their ecto- 

 derm is rich in interstitial tissue, and contains both 

 kinds of thread-cells and palpocils. 



Reproduction may be by 

 fission, and if artificially cut 

 up, all the lost parts are 

 rapidly reproduced, and buds 

 may spring from any part 

 except the tentacles (Fig. ii, 

 D), most frequently from 

 the proximal end, or that 

 nearest the aboral disc. Each 

 bud contains a prolongation 

 of the endoderm and ectoderm 

 and a pouch of the body 

 cavity ; a mouth and one or 

 two nodular tentacles develop 

 at the distal end ; the latter expand and increase in 

 number ; the body cavity at the base of the bud 

 narrows, closes, and the bud falls off (in from three 

 days to six months). Detachment may occur before 

 the aboral cavity is perfectly closed, leaving the 



A. Neuro-muscular cell. 



K. Spermatozoon of Hydra viridis. 



C. Ovum of H. viridis. 



D. H}-dra viridis, showing the for- 



mation of buds. 



* These fibres are more highly refracting when treated with acetic acid 

 than the granular contents of the basis of the ectodenii cells. 



