864 



SPONGES AND ZOOPHYTES 



(fig. 658). 1 The body of the Hydra consists of a simple bag or sac, 

 which may be regarded as a stomach, and is capable of varying its 

 shape and dimensions in a very remarkable degree, sometimes ex- 

 tending itself in a straight line so as to form a long narrow cylinder, 

 at other times being seen (when empty) as a minute contracted 

 globe, whilst, if distended with food, it may present the form of an 

 inverted flask or bottle, or even of a button. At the upper end of 

 this sac is a central opening, the mouth ; and this is surrounded by 



a circle of tentacles or 'arms,' 

 usually from six to teninnumber, 

 w r hich are arranged with great 

 regularity around the orifice. 

 The body is prolonged at its lower 

 end into a narrow base, which 

 is furnished with a suctorial disc, 

 and the Hydra usually attaches 

 itself by this, while it allow r s its 

 tendril -like tentacles to float 

 freely in the water. The wall 

 of the body is composed of two 

 layers of cells ; and between 

 these, which are the ectoderm 

 and endoderm, there is a deli- 

 cate intermediate layer, which 

 forms the supporting lamella.' 2 

 The arms are made up of the 

 same materials as the body ; 

 but their surface is beset with 

 little wart-like prominences, 

 which, when carefully examined, 

 are found to be composed of 

 clusters of ' thread-cells,' having 

 a single large cell with a long 

 spiculum in the centre of each. 

 The structure of these thread - 

 cells or ' urticating organs' will 

 be described hereafter ; at pre- 

 FIG. 658. Hydra fusca, with a young bud sent it will be enough to point 

 at b, and a more advanced bud at c. out t h at fafe apparatus, repeated 



many times on each tentacle, is 



doubtless intended to give to the organ a, great prehensile power, 

 the minute filaments forming a, rough surface adapted to prevent 

 the object from readily slipping out of the grasp of the arm, whilst 

 the central spicule or ' dart ' is projected into its substance, probably 

 conveying into it a poisonous fluid secreted by a vesicle at its base. 



1 On the specific characters of Hydra consult Haacke, Jenaisclie Zeitsclir. xiv. 

 p. 133; and Jickeli, Zool. Anzeig. v. p. 491. 



* To this intermediate layer, Mr. G. C. Bourne applies the term mesoglcea. For an 

 account of its variations and structure among the Coelentera, and a discussion of its 

 homology with the mesoderm of higher Metazoa, see his essay on Fangia in vol. 

 xxvii. of the Quart. Journ. Microsc. 8ci. n.s. 



