TTPE G(ELENTERA. 85 



3. Order Trachymedusse. 



In this order ouly tlie medusa form is present and it re- 

 sembles in many respects that of the Narcomedusse. The 

 bell is somewhat flattened, but is not lobed at the margin ; 

 while the velum has the usual horizontal position and the 

 long slender manubrium projects some distance outside of the 

 subumbrellar cavity. From the gastric cavity four, six, or 

 eight radiating canals arise, which join at the margin the cir- 

 cular canal, and in the ectoderm of the subumbrellar surface 

 over a small area along the line of these canals the reproduc- 

 tive cells develop. At the margin a number of sense-organs, 

 otocysts, are present, agreeing essentially in structure with 

 those of the Narcomedusse and are sometimes projecting or 

 in other cases more or less enclosed in a cavity formed by 

 the growing up around them of the adjacent substance of the 

 bell. The tentacles vary somewhat in shape and structure in 

 different genera. In Shopalonema they are all solid, eight 

 being somewhat longer than the other eight at whose bases 

 are the otocysts ; in Liriope (Fig. 41) four, situated opposite 

 the radiating canals, are hollow and extensible, while the 

 other four are solid and bent back over the exumbrellar sur- 

 face as in the Narcomedusse ; while in Geryonia all the six 

 tentacles are hollow. 



4. Order Campanularise or Leptomedusse. 



In this order both the polyp and the medusa form occur 

 in the life-history of the individual, alternation of generations 

 being the rule. The polyp generation takes the form of a 

 branching colony, which generally shows a certain amount of 

 division of labor among the component individuals. The 

 nutritive hydranths or trophopolyps present the typical form 

 already described (p. 78), and each is seated in a cup of per- 

 isarc, the hydrotheca, into which the polyp may retract. Such 

 polyps reproduce only polyp forms, but in addition others 

 destitute of mouth and tentacles, and known as gonopolyps, 

 occur, which produce by budding numerous medusae or medu- 

 soid buds, the polyp and its progeny being together enclosed 

 in a cup of perisarc, differing in shape from the hydrotheca 



