HYDROZOA AND MEDUSA. 181 



adult jelly-fish ; part of the umbrella-like disc projects downwards 

 in the form of a proboscis (Fig. \h), in the centre of which is a 

 quadrangular mouth, which opens into the digestive sac ; from 

 which arises a series of radiating canals which extend themselves 

 over the disc. The intervals between the segments gradually fill 

 up, so that the divisions are obliterated, and from the borders of 

 the disc sprout forth tendril-like filaments, which hang down around 

 the margin ; while from the four angles of the mouth prolonga- 

 tions are put forth which develop in the adult into four large ten- 

 tacles. They continue to live until the generative organs make 

 their appearance in four chambers, arranged round the stomach, 

 when they produce ova and sperm-cells and die. The fertilised 

 egg, however, does not develop into the large organism by which 

 it was produced, but into the little sex-less Hydra-tuba from which 

 its immediate parent was originally detached ; while the original 

 polypoid body may still remain, and return to its polype-like con- 

 dition, and original mode of increasing by gemmation, forming a 

 new colony, and in time becoming the progenitor of a new series 

 of reproductive Medusae. 



We have here a striking example of the so-called alternation of 

 generation, the phenomena of which are among the most extra- 

 ordinary with which we are acquainted in the whole animal king- 

 dom. The minute, fixed Hydroid polype, in many respects 

 resembling a plant, not more than half-an-inch long, giving rise to 

 the absolutely gigantic free-swimming Medusae, the ova of which, 

 instead of being developed into the likeness of its parent, revert 

 again to the original, tiny, immovable, plant-like organisms from 

 which they were at first produced. 



EXPLANATION OF PLATE XVIII. 



Fig. 1. — Cordylopliora lacustris, showing a polypite and three gono- 

 plieres, in different stages of growth, the largest containing 

 ova. 



,, 2.—Sertularia pinnata, showing capsules. 



,, 3. — tSyncoryne Sarzii, with medusiform zooids (a) budding from 

 between the tentacles. h., Reproductive swimming-bell, 

 detached and free-swimming, m., Manubrium. 



O 



