SECT. in. MEDVSIFORM ZOOIDS. 89 



and by its reaction the zooid is impelled in a contrary 

 direction. From the centre of the bell a stomach hangs 

 down in the form of a proboscis, with a mouth at its ex- 

 tremity, either with or without tentacles and sting-cells. 

 Four canals, or a greater number, which begin in the 

 stomach, radiate through the transparent matter of the 

 bell, and are united by a circular canal round the rim ; 

 they convey the nutritious liquid from the stomach 

 throughout the system. This general structure may be 

 traced in the zooids of the three great families of the 

 oceanic hydraform-zoophytes, in a greater or less degree, 

 from deciduous perfect medusae to such as are imperfect 

 and fixed. 



These medusiform zooids are male and female, and 

 when detached from their parent they are independent 

 creatures, each of them being furnished with nutrient 

 and locomotive organs of its own. They produce ferti- 

 lized eggs, which are developed into ciliated locomotive 

 larvae; after a time these lose their cilia and acquire 

 a rayed sucking disc, with which they fix themselves 

 permanently to a solid object, and, after various changes, 

 each gets a mouth and tentacles and becomes a perfect 

 young hydra. Thus a brood of young hydrse is pro- 

 duced, each of which acquires the compound form of its 

 parent by budding, and as each of these compound 

 animals in its turn gives off medusa-buds, there is a 

 cycle of the alternate forms of hydra and medusa or jelly- 

 fish, showing a singular connection between two animals 

 which seem to have nothing in common. The analogy 

 which so often prevails between plants and animals ob- 

 tains here also, for the medusa-buds bear the same re- 

 lation to the hydra or polype-buds that the flower-buds 

 of a tree do to the leaf-buds : the flower-buds contain 

 the germs of future generations of the tree, while the 

 leaf-buds contain only the undeveloped stems, stalks, and 

 leaves of the individual plant on which they grow. 



