28 THE FRESH- WATER POLYP 



The free end bears a circlet of six or more tentacles, which are hollow pro- 

 longations of the body. At the top of the tube, and surrounded by the 

 tentacles, is a low conical projection, the oral cone or hypostome, in the centre 

 of which is the circular mouth aperture. At the sides of the body projections 

 or bulgings may be seen ; these may be daughter buds at various stages of 

 development, or they may be the developed reproductive organs. The 

 animal is hermaphrodite ; the testis is situated near the mouth end, while 

 the ovary is usually near the "foot " or base of the body tube. 



Notice the clumps or batteries of stinging cells (nematocysts) studding 

 the tentacles. 



Make a drawing of Hydra to show the above features. 



B. Microscopic Structure. 



Examine a longitudinal and a transverse section of Hydra under the low 

 power, and note that the body- wall has the following layers : — 



(a) The ectoderm, a thin layer of many cells packed tightly together, 

 forming a protective covering for the body. 



{h) The endoderm, a thick layer of larger columnar cells lining the . 

 coelenteron or enteric cavity. 



(c) The mesoglcea or supporting lamella, apparently structui<eless, be- 

 tween the ectoderm and the endoderm. 



Note the ccelenteron, enteric or digestive cavity occupying the centre of 

 the body. The mouth leads into the coelenteron, which extends into the 

 tentacles, these being hollow processes, and also into the buds. 



Make a drawing of a longitudinal section and a transverse section of the 

 body. 



Examine the layers of the body-wall under the high power and note the 

 component cells : 



(a) The ectoderm. 



This layer is made up of large ectoderm cells, somewhat conical in shape, 

 arranged .side by side and with their broader ends outwards. Each of these 

 cells has a nucleus and a very large vacuole {not shown in Fig. 11, B and C) ; 

 and the tapering end of the cell is drawn out into crosspiece processes 

 which contain contractile fibrils. These muscular processes lie alongside 

 the mesoglcea and (in trvanserse sections) are just visible as minute dots. 

 The ectoderm cells are therefore myo-epithelial cells. The spaces between 

 the inner and narrower ends of the ectoderm cells are filled up with 

 small and round interstitial cells. Some of the interstitial cells become 

 specialised for reproduction and give rise to the testis (one or more) by 

 a proliferation forming a small conical heap projecting externally and 

 covered by the ectodermal cells. The ovary is similarly formed ; at first 

 it contains several ova, but ultimately only one large ovum remains. 

 Embedded in and between some of the large ectoderm cells there are clear 

 oval sacs with well-defined walls. These are the stinging cells, thread cells, 

 or nematocysts ; and each consists of a tough sac, the neck of which is turned 

 outside in as a hollow pouch. The free end of the pouch is continued into a 



