180 HYDROZOA AND MEDUSAE. 



In some species, as Tiihularia indivisa, the gonophores thus 

 constituted remain permanently attached to the parent organism, 

 but in other cases still further changes ensue. After a time they 

 are detached from the parent, becoming in every respect indepen- 

 dent beings, and are absolutely identical with the organisms 

 commonly called "jelly-fishes," and technically known as Medusce 

 (Fig. 2>^). The essential generative elements — the ova and 

 spermatozoa — are developed in the walls of the radiating tubes 

 which open into the stomach ; and these eggs, instead of pro- 

 ducing young jelly-fish, give origin to the small ciliated infusorian- 

 like body referred to previously, which after a time settles on 

 some solid object at the bottom of the sea, and develops into 

 the primary polypite of a new colony, which again goes through 

 the extraordinary cycle we have been considering. 



In one of the sub-classes of the Hydrozoa — the Lucernariida — 

 a further variation of the reproductive process takes place. Here 

 the free-swimming ciliated embryo (Fig. 4a) attaches itself to some 

 submarine body, forms a mouth at the opposite extremity, around 

 which are developed a row of tentacles (Fig. 4<^), and is now 

 known as a Hydra-tuba. It possesses the power of forming by 

 gemmation large colonies, which may remain in this condition for 

 years, but in which state it is unable to produce the essential organs 

 of reproduction. After a time, however, the body becomes 

 elongated, and exhibits a number of transverse depressions or 

 grooves (Fig. ^c)^ which go on getting deeper and deeper till the 

 whole organism assumes the aspect of a pile of saucers one above 

 the other, with their concave surfaces upwards. At this stage the 

 organism was described by Sars under the name of " Strobila " 

 (Fig. Afd). The edges of these discs become divided into lobes, 

 each lobe presenting a cleft in the centre. The tentacles now dis- 

 appear, and a fresh circle is formed at the base of the Hydra-tube. 

 At last the saucer-like segments drop off one by one, and present 

 themselves in the form of independent, free-swimming Medusoids, 

 under the name of Ephyrce (Fig. \f). They swim about freely, eat 

 voraciously, and increase largely in size; sometimes becoming abso- 

 lutely gigantic — specimens having been found seven feet in diam- 

 eter, with tentacles more than fifty feet in length. As they advance 

 towards maturity they gradually take on all the characteristics of 



