27 



losing it being over, all the former caution and gentleness are laid 

 aside, it is pulled to the polype's mouth with a surprising violence." 



I think as much time as can be spared has now been given to 

 the notice of the tentacles, so we will pass on ; we next see that 

 the outline of the gastric cavity and its extension into tentacles is 

 dark owing to the presence of chlorophyll corpuscles which are 

 contained in the endoderm cells. The ectoderm being transparent, 

 as before stated, gives the light outer border to the parts. At the 

 end of the animal opposite to the mouth is the pedal disc, the ecto- 

 derm cells of which secrete a clear tenacious fluid, emit pseudo- 

 podia by which the animal slowly moves (Pseudopodia are the 

 blunt processes protruded by low forms of life for purposes of loco- 

 motion). 



Now to examine the ectoderm more closely ; we saw befor e that 

 it forms the outer layer of the body wall. It consists of — (1) 

 Covering cells, the surfaces of which are exposed along the body 

 and base irregularly, the latter reaching to the supporting lamina. 

 (2) Epithelial muscle cells, the surface of which is exposed ; the 

 base forming the muscular filament, and is applied to the support- 

 ing lamina. (3) Interstitial cells, which are small irregular 

 masses between the base of (1) i.e., the covering cells. (4) Young 

 cnidoblasts which are deeply placed. (5) Fully formed cnidcblasts 

 which are superficial. In ttie cnidoblast the nemalocyst or thread 

 •cell is developed, the parent cell persisting as a capsule surrounding 

 it, and is produced at its outer or free surface into a small process, 

 the cnidocil. (6' Gland cells, these are restricted to the pedal 

 disc. (7) Ganglion cells, with numerous outrunners continuous as 

 in the endoderm with the cnidoblasts. Sense cells appear to be 

 wanting. 



A supporting lamina separates the ectoderm from the endoderm 

 everywhere except at the mouth. It is delicate, containing fila- 

 ments for both ecto and endodermal muscle cells. The endodermal 

 cells are ciliated, of variable shapes, and during life are amoeboid, 

 t'.e. changing. Endodermal cells, so called, which contain chloro- 

 phyll corpuscles are vacuolated to a certain extent, the vacuoles in 

 some cases being so large as to reduce the protoplasm of the cell to 

 a very thin peripheral lamella 1'hese cells throw out pseudopodia 

 during digestion and also develop in the walls of the gastric cavity 

 muscle filaments which move both circularly and longitudinally. 



Professor Ray Lankester by his Researches on Hydra and 

 Spongilla has clearly established the fact that the green bodies of 

 these animals have not the value of cells. They are devoid of 

 nucleus and membrane. The chlorophyll bodies in Hydra closely 

 resemble the corresponding structures in plants, and appear like 

 them to multiply by fission. They are at one time entirely absent 

 from the ovum, which is the only ectoderm cell that contains them.. 



