114 



CCELENTERATA 



Obelia geniculata 

 (Hargitt). 



Fig. 189— Ch/tia hico- 

 phora (Mayer). 



medusa with IC tentacles at liberation: from South Carolina iforthwards; 

 common on stones, seaweed, etc.; Pacific coast; Europe. 



0. geniculata (L.) (Fig. 188). Colony not more than 30 mm. high 

 and consisting usually of a single zigzag stem bearing 

 alternate hydranths on short annulated stalks; gonan- 

 gia borne in the angles of these stalks; medusa with 

 24 tentacles at liberation: on docks, fucus, etc., from 

 Long Island to Labrador; 

 very common ; California ; 

 cosmopolitan. 



0. gelatinosa (Pallas). 

 Colony tree-like, profusely 

 branching and very large, 

 being sometimes 25 em. high 

 or more, with central stems 

 and zigzag branches; hydro- 

 theeae small, with toothed margin; gonangia small; medusa with 16 

 tentacles at time of liberation: on docks, seaweed, etc., in shallow water 

 from New Jersey to Massachusetts Bay; very common; Puget Sound; 

 Europe. 



2. Clytia Lamouroux. Hydroid colony sparsely branched or not 

 at all, the hydranths being at the end of a usually long, more or less 

 annulated stalk which rises from the hydrorhiza; hydrotheca with 

 toothed margin; gonangia often annulated, on the hydrorhiza or the 

 stem; medusa with 16 tentacles and 16 lithocysts: 8 species. 



C. bicophora Agassiz (Fig. 189). Colony about 10 mm. high; medusa 

 5 mm. in diameter, hemispherical when liberated but later becoming 



more flattened; gonads brown, manubrium 

 short, with 4 small oral lobes: from South 

 Carolina to Arctic Ocean, on fucus, docks, 

 etc., in shallow water; often common. 



3. TiAROPSiS Agassiz. Hydroid form un- 

 known; medusa hemispherical; marginal ten- 

 tacles very numerous in adult; manubrium 

 short with frilled mouth opening; 8 lithocysts 

 above each of which is an ocellus : 6 species. 

 T. diademata Agassiz (Fig. 190). Medusa 

 sloping sides; manubrium with 4 prominent 

 often abundant. 



Hydroid form unknown; medusa hemi- 



Fig. 100 — Tiaropsis diade- 

 mata (Ma^'er). 



15 mm. in diameter, with 

 lips: New England coast; 



4. Etjcheilota McCrady. 

 spherical; tentacles each with a pair of lateral cirri at its base: 6 

 species. 



