156 



ZOOLOGY 



SECT. 



position. In the Narcomedusse the manubrium is short ; in the 

 Trachymedusae it is always well developed, and is sometimes (Fig. 

 113, 2) prolonged into a long, highly contractile peduncle, having 

 its inner surface produced into a tongue-like process (tg) which 

 protrudes through the mouth. In some the gastric cavity is 

 situated in the manubrium, which in such a case is looked upon as 

 partly of the nature of a process of the sub-umbrella (pseudo- 

 manubrium). 



The simplest case of the development of Trachylinae is seen in 

 dfiginopsis, one of the Narcomedusae. The oosperm gives rise to 

 a ciliated planula, which forms first two (Fig. 116), then four 

 tentacles, and a mouth, hypostome, and stomach. The larva of 

 ^Eginopsis is thus a hydrula, closely resembling the corresponding 

 stage of Tubularia. After a time the tentacular region grows out, 

 carrying the tentacles with it, and becomes the umbrella of the 

 medusa. Thus the actual formation of the medusa from the 



hydrula of 

 ^Eginopsis 

 c o r r e sponds 

 precisely with 

 the theoretical 

 derivation 

 given above 

 (p. 137). It 

 will be seen 

 that in the 

 present case 

 there is no 

 metagenesis or 

 alternation of 



generations, but that development is accompanied by a metamor- 

 phosis that is, the egg gives rise to a larval form differing in a 

 striking manner from the adult, into which it becomes converted 

 by a gradual series of changes. 



Metagenesis is, however, not quite unknown among the Trachy- 

 linae. In a parasitic Narcomedusa (Cunina parasitica) the planula 

 fixes itself to the manubrium of one of the Trachymedusae which 

 serves as its host, and develops into a hydrula. But the latter, 

 instead of itself becoming metamorphosed into a medusa, retains the 

 polype form and produces other hydrulae by budding, these last 

 becoming converted into, medusae in the usual way. 



ORDER 3. HYDROCORALLINA. 



The best-known genus of Hydroid Corals is Mittepora, one species 

 of which is the beautiful Elk-horn Coral, M. alcicornis. ' The dried 

 colony (Fig. 117, A) consists of an irregular lobed or branched mass 



FIG. 116. Larva of JEginopsis. m. mouth'; t. tentacle. 

 (From Balfour, after Metschnikoff.) 



