156 



ZOOLOfJY 



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 

 JEginopsis, 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 

 x^ginopsis is thus a Tiydnda, 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 



h y d r u 1 a of 

 JLginopsis 

 c o r r e spends 

 precisely with 

 the theoretical 

 derivation 

 given above 

 (p. 137). It 

 will be seen 

 that in the 



FIG. 116. Larvaof JEginopsis. m. mouth"; t. tentacle. 

 (From Balfour, after Mctschnikoff.) 



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 Millepora, one species 

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

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



