THE CCELENTERATA 155 



of a sensory nature. They are long thread-like cells situated 

 between the epithelio-muscular cells and branching out at their 

 inner ends like the nerve cells, to which no doubt they are closely 

 related. 



4. Lastly, it is to the interstitial cells that we have to look for the 

 primitive germ cells. Hydra is hermaphrodite, that is to say, the one 

 individual produces both spermatozoa, the male cells, and ova, the 

 female cells. We express the same idea when we term it monoecious, 

 i.e. of one sexual form as opposed to most of the higher animals, 

 which are dioecious, i.e. of two forms, male and female. The 

 spermatozoa are aggregated in large numbers, producing one or 

 more enlargements, the testes, lying in the oral third of the body 

 beneath the ring of tentacles. The ova when fully ripe are very 

 large single isolated cells, which also, produce swellings, the ovaries, 

 situated about the middle of the body below the testes. 



The entoderm of Hydra is composed in the main of three types of 

 cells : 



1. Again, the main part is composed of musculo-epithelial cells 

 which are, however, much larger and more columnar than those of 

 the- ectoderm. Their muscular processes are also embedded in the 

 mesoglea, but are arranged transversely to the long axis of the body, 

 i.e. circularly, and are not so strongly developed. The edge of the 

 cell towards the gastral cavity is of indefinite shape and capable of 

 putting forth amoeboid processes which can seize small pieces of the 

 food. The cytoplasm is very vacuolated, sometimes containing one 

 large vacuole, and also granular with tiny particles of food in it. 

 In H. viridis and H. oligactis there are also present in the cell 

 numerous minute ovoidal capsules of a bright green or yellow colour, 

 according to the species, lying towards the basal part of the cell. 

 They are really minute plants, Algae, that have made these cells 

 their dwelling-place. A noticeable vesicular nucleus with a distinct 

 nucleolus is also present, and usually more or less basally situated. 



2. The second type of cell is secretory, and such glandular cells 

 are to be found fairly generally scattered, but most commonly in 

 the neighbourhood of the hypostome. They are much like the other 

 cells, but contain large droplets of a slimy or albuminous nature. 



3. Lastly, we find in the entoderm a certain number of inter- 

 stitial cells wedged in between .the bases of the epithelio-muscular 

 cells, but they are not nearly so plentiful as in the ectoderm, and 

 only occur here and there. 



The limited movement of Hydra as a whole, and that of 

 the tentacles, has already been mentioned. Its feeding is of an 

 interesting type. When the prey reaches the gastral cavity it comes 

 under the action of the secretion of the gland cells which contain 



