SPONGES AND ZOOPHYTES. 123 



( 515). This great division includes the two principal groups, the HY- 

 DROZOA and the ACTINOZOA ; the former comprehending the Polypes, and 

 the latter the Anemonies. In the Hydrozoa there is no separation be- 

 tween the digestive cavity and the external body-wall; and the reproduc- 

 tive organs are external. In the Actinozoa the wall of the digestive sac 

 is separated from the external body-wall by an intervening space, which 

 communicates with it, and must be regarded as an extension of it; and 

 this is subdivided into chambers by a series of vertical partitions, to 

 which the reproductive organs are attached. As most of the Hydrozoa 

 or Hydroid Polypes are essentially Microscopic animals, they need to be 

 described with some minuteness; whilst in regard to the Actinozoa those 

 points only will be dwelt-on, which are of special interest to the Micro- 

 scopist. 



514. HYDROZOA. The type of this group is the Hydra or fresh-water 

 polype, a very common inhabitant of pools and ditches, where it is most 

 commonly to be found attached to the leaves or stems of aquatic plants, 

 floating pieces of stick, etc. Two species are common in this country, 

 the H. viridis or green Polype, and the H. vulgaris, which is usually 

 orange-brown, but sometimes yellowish or red (its color being liable to 

 some variation according to the nature of the food on which it has been 

 subsisting); a third less common species, the H. fusca, is distinguished 

 from both the preceding by the length of its tentacles, which in the 

 former are scarcely as long as the body, whilst in the latter they are, 

 when fully extended, many times longer (Fig. 354). 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 extending 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 sur- 

 rounded by a circle of tentacles or f 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 fur- 

 nished with a suctorial disk; 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 cells imbedded in sarcode-substance; 

 and between its two layers there is a space chiefly occupied by undiffer- 

 entiated sarcode, having many 'vacuoles' or 'lacunae' (which of ten seem 

 to communicate with one another) excavated in its substance. 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 ( 528); at 

 present it will be enough to point-out that this 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, prob- 

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

 The latter inference is founded upon the oft-repeated observation, that 

 if the living prey seized by the tentacles have a body destitute of hard 

 integument, as is the case with the minute aquatic Worms which consti- 



