6 



events, I propose to describe them first for the purpose of this 

 paper, as this course will enable me to describe the various forma- 

 tions of the polypary in a simpler manner. 



There are four species of fresh-water Hydra — viz. Hydra viridis, 

 H. vulgaris, H. holigadis or fusca, and H. atlenuata. 



Their bodies are composed of gelatinous and contractile tissue. 

 A general description would be : a semi-transparent tube possessing 

 an adhesive base at one end, enabling it to attach itself to anything ; 

 at the other extremity of the tube is the mouth, surrounded by 

 tentacles, which are capable of enormous elongation, as also is the 

 body of the animal. They are armed by formidable thread-cells, 

 which are grouped together, and have the power of throwing out 

 stinging organs for killing or paralysing their prey. A small ento- 

 mostraca or annelid coming into contact with the tentacle stands a 

 small chance of escape, for these stinging threads are projected in 

 immense numbers ; even if it should struggle away and escape, it 

 dies soon after. 



The Hydra; are propagated in two ways — by gemmation, and by a 

 true sexual reproduction. 



In the spring of the year young Hydras are projected from the 

 body of the parent, sometimes in large numbers, and grow to a fair 

 size before leaving the mother, causing her to look like a plant with 

 branches growing out of the main stem. After a time they detach 

 themselves, and become independent animals. 



Towards the autumn true reproductive organs are developed, 

 usually on the same individual. 



The ovary is a simple sac (formed by the outer walls of the body) 

 in which the ovum is developed. 



The spermaries are conical projections, developed just at the base 

 of the tentacle. 



The eggs are developed late in the autumn ; they break away from 

 the parent and undergo no change until the following year. 



The Hydra possess enormous vitality ; they may be cut up into 

 many portions, and each portion is capable of producing a perfect 

 Hydra. They are very common, and are found, attached to weed, 

 in most stagnant ponds and ditches. 



Hydra viridis is (as its name implies) of a lovely, pale green colour, 

 and forms a charming object under a low power of the microscope 

 when illuminated on a dark ground. If alarmed it shrinks up into a 

 small green body, and is scarcely visible on the weed. 



This description will apply to most of the Hydroid Zoophytes, with 

 this difference, that the marine forms are clothed with a chitinous 

 covering (the polypary), and the base of each polypite. instead of 

 being a sucker, is prolonged into the polypary. 



The flesh-like trunk or ccenosarc is united to its neighbours — in 

 fact, is an outgrowth from the original polypite, which attached itself 

 when tired of swimming about freely. The whole colony has 

 developed by successive buddings, and, like a plant, each branch is 



