TTPE CWLENTERA. 83 



the ccelenteron ciliated ; in the tentacles, which may be either 

 hollow or solid, it may retain its ciliated character, or else in 

 the solid tentacles form a solid axis similar to that of the 

 tentacles of the polyp forms. 



Such are the general structural features common to all 

 the Hydromedusse. It remains to add here certain points by 

 which especially they are distinguished from other groups of 

 Cnidaria. These are : {a) the ectoderm and endoderm meet at 

 the mouth-opening ; {b) the sense-organs when present are 

 never modified tentacles : (c) a velum is always present in the 

 medusa forms ; and {d) the reproductive elements have their 

 origin in the ectoderm. 



1. Order Hydrariae. 



To this order belongs the Hydra, a common inhabitant of 

 fresh-water ponds and streams in all parts of the world. It 

 is a simple cylindrical organism, adherent by one extremity 

 to foreign bodies, though not fixed, being able slowly to 

 change its position. Below the short hypostome is a single 

 row of exceedingly extensible tentacles, which are hollow, a 

 peculiarity found in no other Hydromedusan polyps. So long 

 as conditions are favorable and nutrition abundant Hydra 

 reproduces by budding, the buds separating from the parent 

 form after a certain period of growth. Sexual reproduc- 

 tion also occurs, the spermatozoa developing in the ectoderm 

 a little below the ring of tentacles, while the ova form in the 

 same layer somewhat lower down on the bodj', the animals 

 being hermaphrodites. The ova after fertilization remain 

 imbedded in the ectoderm of the parent for some time, but 

 later, developing around them a cyst, they sink to the bottom 

 of the water, and there remain usually for some time without 

 developing further. In this condition they are able to with- 

 stand a considerable amount of cold and dryness, and so 

 may tide the species over unfavorable conditions. No medusa 

 stage occurs in Hydra. 



Hydra grisea L. is a brown form, relatively large in size, while H. 

 viridis L. is smaller and of a dark-green color due to chlorophyll-contain- 

 ing corpuscles imbedded in the ectoderm-cells and supposed by some 

 authors to be unicellular Algse of a symbiotic habit. (See p. 20.) 



