84 INVERTEBRATE MORPHOLOGT. 



2. Order Narcomedusae. 



In the HydrarisB the polyp form occurs without a medusa, 

 in the order Narcomedusce the reverse is the case, since in it 

 the medusa form is the only one present in a typical state of 

 development. The medusse are usually somewhat lens- 

 shaped structures (Fig. 42), with a lobed margin, the velum 

 {v), instead of being horizontal, being pendent from the mar- 



PiG. 43. — Gunoetaniha oclonaria Haeck. (After Bhooks). 

 TO = margin of bell. ot = otocyst. v — velum. 



gin (m) and extending up in the intervals between the lobes. 

 At the apex of each one of these intervals is situated a short, 

 stiff, solid tentacle which is usually bent backward over the 

 exumbrellar surface and is tipped by a knob of nematocysts. 

 The cavity of the short manubrium leads into a gastric 

 chamber which is prolonged out towards the margin into broad 

 pouches which lie opiDosite the tentacles and the intervals be- 

 tween the lobes, and around the margin, following the edge of 

 the lobes and therefore having a festooned arrangement, runs 

 a narrow circular canal which communicates with each radial 

 pouch at the apex of each interlobular interval. This struc- 

 ture is, however, absent in the American species Cunoctantha 

 octonaria. The reproductive organs develop in the subum- 

 brellar ectoderm covering the pouches and sometimes extend 

 on to the manubrium. Around the margin of the lobes are 

 seated club-shaped projecting otocysts {ot) composed of an 

 external layer of ectoderm surrounding a number of endo- 

 dermal cells, one or more of which contain a crystal of 

 carbonate of lime. The ectoderm cells in the neighborhood 

 are largely sensory and provided with long cilia, their inner 

 ends contributing to the formation of the marginal nerve- 

 ring. 



