128 PHYSIOLOGY. 



PHYSIOLOGY OF THE HYDROIDA. 



In attempting a classification of the various phenomena of hydroid life, we are at once met 

 by the difficulty of arranging them under definite physiological heads. The low grade of speciali- 

 sation on which the Hydroida stand, renders it often impossible to assign to these phenomena a 

 definite physiological significance, and many of the acts which make up the life of such simple 

 organisms can scarcely claim to be referred to one more than to another of the great classes under 

 which the functions of animals are usually distributed. 



Notwithstanding this, however, we shall find it convenient to speak of the vital acts of the 

 Hydroida under the following heads : — 1. Digestion. 2. Circulation, Nutrition and Growth of 

 the Tissues, and Respiration. 3. Secretion. 4. Contractility. 5. Sensation. 6. Phosphores- 

 cence. 7. Reproduction. We must, however, keep in mind that these classes do not all neces- 

 sarily possess the definiteness which characterises them in the higher and more specialised 

 members of the animal kingdom. 



1. Bigestion. 



So far as observation has taught us anything regarding the food of the Hydroida, we may 

 conclude that it consists mainly of living animals, though Diatomaceae and other minute free 

 vegetables contribute also to their subsistence. The patient observations of Trembley and the 

 older naturalists on the fresh-water Hydra had long ago shown that animals of considerable size, 

 such as entouiostraca, and even aquatic larvae and worms, are seized by the tentacles of this 

 voracious little hydroid, and carried to the mouth, through which they are borne into the cavity 

 of the body ; and subsequent observations have proved that in other Hydroida the nature of the 

 food and the capture of the prey are in all essential points similar to what has been noticed in 

 the Hydra . 



It is in this act that the functions of the thread-cells seem to be manily called into play ; 

 and repeated observations have shown that no sooner do the tentacles come in contact with the 

 living prey, than all power of resistance in the latter is at an end ; its efforts to escape seem 

 suddenly paralysed, and it becomes an easy victim to the rapacity of its captor. 



If the prey be at this stage released from the grasp of the hydroid and placed under the 

 microscope, all the soft tissues of its surface will be found pierced with discharged thread-cells. 



Observers are by no means agreed as to the true functions and mode of action of the thread- 

 cells. By most they are supposed to penetrate the tissues of the prey, and those which, like the 

 larger thread-cells of Hydra, are provided with an apparatus of barbed spines, have been described 

 as plunged beyond the barbs into the soft tissues of the victim. Other observers, and more 



