HYDROZOA AND MEDUS.E. 179 



which occupies the whole length of the polypite, and at its lower 

 extremity is continuous with the tubular cavity which is excavated 

 throughout the centre of the coenosarc ; and hence the nutritive 

 particles obtained by each polypite serve for the support of the 

 whole colony. 



From this original Hydraform polype, a stalk of coenosarc 

 grows upwards, and on this stalk new polypites are developed ; 

 thus giving rise to a more or less arborescent, plant-like colony. 

 One of the best known forms, which is familiar to all visitors at 

 the seaside, is the Sertularla, or sea-fir (Fig. 2), which, by those 

 unacquainted with its true nature, is almost always set down as a 

 seaweed. They are entirely confined to the sea, with the single 

 exception of Cordylophora, which inhabits fresh water (Fig. i). 



These colonies continue to increase for some time by gemma- 

 tion, but the polypites thus produced can only remain attached to 

 the original individual, and are unable to start new colonies. For 

 this purpose it is necessary that a special form of polypite should 

 be developed, entirely devoted to the purposes of reproduction. 

 These reproductive gemmae, which are totally different from the 

 nutritive zooides, both in structure and function, bud forth from 

 the base of the tentacles. In the simplest form they appear as 

 mere protuberances from the external wall of the Hydrozoon 

 (Fig. i^), forming a sort of sacculated pouch, attached by a short 

 stalk to the parent colony, which after attaining a certain size 

 develop ova and sperm-cells. In Sertularia they are developed 

 in chitinous receptacles, known as " gonotheca" (Fig. 2), and 

 remain permanently attached to the parent colony. In the Corynida 

 and Ca??ipa?mlarida, the reproductive elements are developed in 

 distinct buds or sacs, which are external processes of the body- 

 wall, and are termed " gonophores " (Fig. 3^). Each gonophore 

 develops into a little transparent, glassy, bell-shaped disc, attached 

 by its base to the parent organism : from its roof, like the clapper 

 of a bell, there depends a peduncle or " manubrium " (Fig. 3;//) ; 

 while from the rim hang a row of long and delicate tentacles. 

 In the manubrium is formed a mouth, which opens into the 

 stomach, from which four tubes radiate to the margin of the bell, 

 where they communicate with each other by a single circular 

 canal, which surrounds the disc. 



