864 



SPONGES AND ZOOPHYTES 



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

 u liich may lie 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 

 ylohe, 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 ten in number, 

 which 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 allows 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. - 

 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 iirticating organs' will 

 be described hereafter; at pre- 

 sent it will be enough to point 

 out that this apparatus, repeated 

 many times on each tentacle, is 

 organ a great prehensile power, 



FIG. 658. Hydra f iiitca, with a young bud 

 at b, and a more advam-ed bud at c. 



to the 



doubtless intended to " 



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 bv a vesicle at its base. 



1 On the specific characters of Hi/dm consult Haacke, Ji-nainchi: Zritschr. xiv. 

 p. l:'.:!; and Jickdi, /.iiol. Anzcig. v. p. 4!H. 



' To this intermrdiiitc' layer, Mr. G-. C. Bourne applies the term niesof/Jten. For an 

 arc-omit of its variations and structure among the Cu;leiitera, and a discussion of its 

 limnology with the mesoderm of higher Meta/.oa, see his essay on Fitngin in vol. 

 xxvii. of the' ( t h<ii>-/. Jmirii. Mirt'uai-. St'i. n.s. 



