118 CCELENTERATA 



Key to the families of Trachomedusae here described : 



i Radial canals 4 or 6. 



&! Gonads not plate-like, usually undulating 1. PETASIDAE 



5, Gonads plate-like 3. GEBYONIIDAE 



03 Radial canals 8 2. AGLAUBIDAE 



FAMILY 1. PETASIDAE. 



Trophosome: minute and apparently rudimentary, so far as known, 

 probably wanting in many cases. Gonosome : medusa with 4 or 6 radial 

 canals; gonads elongate and much folded or sac-like; tentacles either 



Fig. 198 Gonionemus murlachi. A, hydroid (Perkins) ; B, medusa (Mayer). 



with or without a pad-like cluster of modified nettle cells near the distal 

 end for purposes of adhesion; manubrium short: about 14 genera. 



1. GONIONEMUS.* A. Agassiz. Trophosome: minute, so far as known. 

 Gonosome : medusa with 4 radial canals, along the entire length of which 

 the sinuous gonads extend; adhesive pad near the extremity of each 

 tentacle; numerous Iithoc3 r sts present: 7 species; cosmopolitan. 



G. murbachi Mayer (Fig. 198). Trophosome: solitary hydra-like 

 individuals 1 mm. high with 4 tentacles. Gonosome: medusa 20 mm. in 

 diameter and half as high; marginal tentacles from 16 to 80, long and 

 stiff and green at the base; gonads brown; manubrium short with 4 

 frilled oral lobes: Vineyard and Long Island Sounds. 



G. vertens A. Ag. Similar to the above, but higher than wide: 

 Pacific coast from Washington to Alaska. 



2. MICROHYDRA f Potts. Trophosome : a minute hydroid without 

 tentacles and solitary, but multiplying by lateral budding. Gonosome: a 



* See "The Development of Gonionema murbachii," by H. F. Perkins, Proc. Acad. 

 Nat. Sci., 1902, p. 750. 



t See "On the Medusa of Microhydra," etc., by Edward Potts, Quart. Jour. Mic. 

 Sci., Vol. 50, p. 623, 190G. "Microhydra in 1907," Proc. Delaware Co. Inst., Vol. 3, p. 

 89, 1908. 



