compound hydrozoa: — CORYNE. 537 



has no further participation in it, than has the Fucus in the analo- 

 gous fertilization of its germ-cells after their discharge (§ 258). 

 Although the production, from such an egg, of a new Hydra, 

 similar in all respects to its parent, has not yet been witnessed, 

 there seems no reason to doubt the fact. It would seem that this 

 alternation in the method of Reproduction, between the Gemmi- 

 parous and the Sexual, is greatly influenced by external tempera- 

 ture ; the Eggs being produced at the approach of winter, and 

 serving to regenerate the species in the spring, the parents not 

 being able to survive the cold season ; whilst the Budding-process 

 naturally takes-place only during the warmer part of the year, 

 but may be made to continue through the whole winter by keeping 

 the water inhabited by the polypes at a sufficiently high tempera- 

 ture. — The Hydra possesses the power of free Locomotion, being 

 able to remove from the spot to which it has attached itself, to 

 any other that may be more suitable to its wants ; its changes of 

 place, however, seem rather to be performed under the influence 

 of light, towards which the Hydra seeks to move itself, than with 

 reference to the search after food. 



413. Some of the simpler forms of the Compound Hydrozoa 

 may be likened to a Hydra whose Gremmfe, instead of becoming 

 detached, remain permanently connected with the parent ; and as 

 these in their turn may develope gemma? from their own bodies, a 

 structure of more or less arborescent character may be produced. 

 The form which this will present, and the relation of the compo- 

 nent Polypes to each other, will depend upon the mode in which 

 the gemmation takes-place ; in all instances, however, the entire 

 cluster is produced by continuous growth from a single individual ; 

 and the stomachs of the several polypes are united by tubes, 

 which proceed from the base of each, along the stalk and branches, 

 to communicate with the cavity of the central stem. — This is the 

 case with the family Corynidos, which are composite fabrics, 

 sometimes quite arborescent in form, but unpossessed of any firm 

 investment, the external wall being only strengthened by a thin 

 horny cuticle. A very beautiful Marine species of this family 

 (the Coryne pusilla) is common on Sea- weeds and stones between 

 tide-marks ; sometimes clustering parasitically round the stalks of 

 Tubularia so as to form a thick beard-like mossiness ; each aggre- 

 gate structure, however, not being more than an inch in length. 

 The Tentacles (as in Fig. 276, a) are short, and arise from the 

 whole surface of the body of the polype, instead of from the 

 margin of the mouth alone ; and at first it seems difficult to 

 understand how they can be of service in bringing food to the 

 mouth, which is situated at the very extremity of the branch. 

 Observation of the living animal, however, soon removes this 

 difficulty ; for the head is so very flexible, that the mouth can 

 bend itself down towards any of the tentacles which may have 

 entrapped prey, all its movements being performed in a very 



