CHAPTEE XI 



HYDKOZOA {CONTmUED) : TKACHOMEDUSAE NAECOMEDUSAE 



SIPHONOPHOEA 



Order VII. Trachomedusae. 



The orders Trachomedusae and Narcomedusae are probably closely 

 related to one another and to some of the families of Medusae at 

 present included in the order Calyptoblastea, and it seems 

 probable that when the life-histories of a few more genera are 

 made known the three orders will be united into one. A^ery 

 little is known of the hydrosome stage of the Trachomedusae, but 

 Brooks ^ has shown that in Liriope, and Murbach ^ that in 

 Gonionema, the fertilised ovum gives rise to a IIijd7'a-like form, 

 and in the latter this exhibits a process of reproduction by 

 gemmation before it gives rise to Medusae. Any general state- 

 ment, therefore, .to the effect that the development of the 

 Trachomedusae is direct would be incorrect. The fact that the 

 hydrosoraes already known are epizoic or free-swimming does not 

 afford a character of importance for distinction from the Lepto- 

 medusae, for it is quite possible that in this order of Medusae 

 the hydrosomes of many genera may be similar in form and 

 habits to those of Liriope and Gonionema. 



The free border of the umbrella of the Trachomedusae is 

 entire ; that is to say, it is not lobed or fringed as it is in the 

 Narcomedusae. The sense-organs are statocysts, each consist- 

 ing of a vesicle formed by a more or less complete fold of 

 the surrounding wall of the margin of the umbrella, containing 

 a reduced clapper-like tentacle loaded at its extremity with a 



' " Life-History of the Hydromedusae," Mem. Boston Soc. iii. 1885, p. 359. 



^ Journ. Morph. xi. 1895, p. 493. 



288 



