96 SYNOPTIC COLLECTION. 



H YDROPHORA. NARCOMEDUSAE. 



The Narcomedusae are a group of living hydroids 

 which throw considerable light on the evolutionary history 

 of the Hydrozoa, as pointed out by Brooks. 1 PI. 112, 

 figs. 1-19, represents the development of Aeginopsis 

 from the egg to the hydra stage. Fig. i is the fresh laid 

 egg; fig. 2, the two celled stage ; fig. 3, the beginning of 

 the second furrowing stage ; fig. 4, the four celled stage ; 

 fig. 5, the beginning of 'the third furrowing stage; fig. 6, 

 the eight celled stage ; and fig. 7, the beginning of the 

 fourth furrowing stage ; fig. 8 is the sixteen celled embryo 

 in cross section; fig. 9, the embryo composed of about 

 thirty-two cells. Figs. 10 and 1 1 show isolated cells of the 

 same embryo; fig. 10 is the endoderm cell just formed, 

 and fig. ii, the completed or finished endoderm cell. 

 PMg. 12 is the embryo of fifty-nine cells ; fig. 13 is the two 

 layered larva; fig. 14, the larva lengthened; fig. 15, the 

 further developed l#rva; figs. 16 and 17, cells of the same 

 (16, cell of ectoderm; 17, of endoderm) ; fig. 18 is the 

 larva with two tentacles and no mouth; fig. 19, the larva 

 of the fourth day- It is now a hydra with a shortened 

 body, a mouth at the end of a long oral cone, at the base 

 of which are tentacles. Unfortunately Metschnikoff did 

 not figure the further development of the hydra into the 

 medusa. The tentacular zone, however, grows out into 

 an umbrella which carries the tentacles with it ; sense 

 organs and a veil or velum are soon acquired, and the 

 hydra becomes converted into a medusa. Its parts can 

 be plainly traced in the medusa, and the difference in 

 external appearance is due mainly to the great develop- 

 ment of the middle layer or mesoderm which forms the 

 umbrella. If now we could find a genus where the larva, 

 while yet a hydra, should fasten itself to some object, 



iMem. Boston Soc. Nat. Hist., Ill, no. 12, 1886. 



