24 HYDRA 



B. Transverse Sections. 



Examine a series of transverse sections through the body 

 of Hydra. To prepare these hill the animal in the expanded, 

 condition by suddenly irrigating it with osmic acid; wash 

 it well in distilled water ; transfer to alcohol; stain with 

 picrocarmine, and then imbed in paraffin and ciit with 

 a microtome. 



1. The ectoderm. 



a. The large ectoderm cells are well seen in such 



sections ; their muscular tails, which are mainly 

 longitudinal in direction, are cut transversely, 

 and appear as a row of highly refractive dots 

 on the outer surface of the supporting lamella. 



At the basal end or foot of the Hydra the 

 ectoderm cells are more columnar in shape, 

 contain longitudinal rows of granules, and are 

 probably glandular. 



On the outer surface of the ectoderm cells 

 there is a thin cuticular layer, more obvious in 

 the brown than in the green Hydra. 



b. The interstitial cells are well seen in sections 



through the body or tentacles, but are absent in 

 the foot. 

 e. The nematocysts are very abundant in the tentacles, 

 less numerous in the body, and absent in the foot. 

 In specimens killed with osmic acid, few, if any 

 of them will be discharged. 



2. The supporting lamella is seen in transverse sections as 



a very thin transparent layer, between the ectoderm 

 and entoderm. 



3. The endodenu ceUs vary greatly in shape in different 



parts of the body and at different times. During 

 life they are amoeboid, and may extend so far inwards 

 as to almost obliterate the cavity of the animal. 

 They contain vacuoles, which are often so large 

 that the cell-substance is reduced to a thin peripheral 



