218 



CCELENTERATA 



which the gonophores arise from the blastostyle, a specialized polyp, without 

 mouth or tentacles (fig. 171, /). The typical Campanulariae produce medusae, 

 while some forms, like Thaumantia* and sEquoria* have no hydroid stage; on 

 the other hand, Sertularia* and Plunmlaria* have no medusa stage. Other 

 common genera, Clytia* Diphasia* and Aglaophenia* among hydroids; 



Obelia* Tima* Rhegmatodes* among me- 

 dusae. Possibly the fossil GRAPTOLITES be- 

 sb long near here. Only the perisarc is known; 



this has hydrothecae, in which it is supposed 

 the hydranths occurred. 



Order V. Trachymedusae. 



These medusas, mostly from warmer seas, 

 have no hydroid stage. The characters are 

 given on p. 216, Trackynema,* Liriope* (fig. 

 1 80), and Campanella* in our waters, 

 Geryonia, etc., in Europe. 



.fl 



Order VI. Narcomedusae. 



In addition to the characters on p. 217 

 may be added that the tentacles arise from 

 the outside, above the rim of the bell. 

 Cunocantha* (fig. 180), Cunina*, sEgina. 

 The larvae frequently live as parasites on 

 other medusae, and they may be able to re- 

 produce asexually, forming sacs in which 

 new medusae are budded. 



Order VII. Siphonophora. 



t 



FIG. 181. Diagram of Siphono- 

 phore (from Lang). A-H, groups 

 of different individuals; ds, cover- 

 ing scales; go, gonophores; hy, feed- 

 ing polyps; p, 'feelers' (digestive); 

 sb, float; sg, swimming bell (necto- 

 calyx) ; st, stalk. 



The Siphonophora are among the 

 most beautiful of pelagic animals, some 

 transparent, some brightly colored. 

 Each (fig. 181) consists of a colony of 

 individuals springing from a common 

 cosnosarcal tube which is strongly mus- 

 cular and contains a central canal, lined 

 with entoderm, by which the members 

 of the colony receive their nourishment. 

 At one end the tube is usually closed by 

 a float of invaginated ectoderm, filled 

 with air, the pneumataphore, which keeps 



the colony vertical in the water. 

 The individuals, springing from the ccenosarcal axis, perform different 

 functions and hence differ in structure. Close behind the float commonly 

 come several swimming bells (nectocalyces) which retain only those medusan 

 structures (bell, velum) necessary for swimming and those (ring and 



