HYDRO1DEA. 337 



piece of weed with the oral end downwards, a position ordinarily assumed 

 by it during life. It is much enlarged, its natural size being at the greatest, 

 in a fine example, barely f of an inch. 



Hydra differs from the hydroid form of other Craspedota or Hydrome- 

 dusae, (i) in being free and locomotive ; (3) in being only temporarily 

 colonial, as the buds formed are eventually set free from the parent and 

 from each other ; (3) in being a sexual Hydroid form and not developing a 

 dimorphic sexual zooid or Medusa ; (4) in being naked, i. e. devoid of a 

 perisarc. The marine and freshwater genus Protohydra agrees with Hydra 

 in being locomotive and non-colonial. 



a. Tentacle. The tentacles are contracted. They form a single circle : 



in number they vary from six to ten as a maximum in this species, but 

 the odd numbers, seven and nine, are often met with. They are hollow 

 and their cavities are continuous with the gastric cavity. 



b. Hypostome, or oral cone. It is conical when protruded, and the mouth 



is situated at its apex. 



c. The line points to the gastric cavity. The outline of this cavity and its 



extension into the tentacles is dark, owing to the chlorophyl corpuscles 

 contained in the endoderm cells. The ectoderm cells, on the contrary, 

 are transparent, and hence the light outer border both to body and 

 tentacles. The oral and stomachal regions constitute the hydroce- 

 phalis. The peduncle is known as the hydrocope. 



d. Testis. The testes are always placed near the base of the tentacular 



circle, and are, as here, generally more than one in number (8-9). 

 They are formed by the local multiplication of interstitial ectoderm 

 cells, which are developed into spermatozoa and raise the covering-cells 

 into a pointed capsule. This capsule dehisces at its apex, setting 

 free its contents into the water. 



e. Ovum. The ovum is as a rule solitary. It is produced by the growth 



of one out of a mass of cells produced by the division of interstitial 

 ectoderm cells. The outer cells undergo regressive changes, and the 

 products formed serve as food to the growing ovum. The ovum is 

 the only ectoderm cell which forms chlorophyl corpuscles, and it may 

 be noted that these bodies have colourless predecessors. The covering- 

 cells are raised by the ovum and burst The ovum is then fertilised, 

 and undergoes fission : the superficial cells are completely used up in 

 this species but not in H.fusca to form (i) a chitinous coat, (2) a vitel- 

 line membrane, and (3) a mucous coat. Thus protected, the ovum 

 drops away from the parent and remains quiescent. 



/. Adhesive or pedal disc. The ectoderm cells of this disc are glandular 

 and secrete a clear tenacious fluid, and they emit pseudopodia, and 

 hence move the animal slowly. Hydra swims by creeping up to the 

 surface of the water and exposing its disc ; and it creeps by bending 



