COELENTERATA. 



109 



COC..V. 



by means of small suckers on their tentacles (Clavatella prolifera, Pectanthus 

 asteroides), or can adhere by the suctorial action of their mouth openings 

 (Pelagia), and even in rare cases are able to lead a parasitic life (Mnestra 

 -//arasitica on a pelagic snail Phyllirhoe bucephala}. 



v 



The contraction of the bell is effected by the circular muscles of 

 the sub-umbrella surface. The ejection of water caused by this 

 contraction drives 

 the medusa along. 

 The velum is a mus- 

 cular membrane at 

 the edge of the bell. 

 It consists of a fold 

 of ectoderm, and 

 assists in the move- 

 ment of the medusa 

 by elongating the 

 umbrella cavity and 

 narrowing its aper- 

 ture. When the 

 velum is absent the 

 margin of the bell 

 is lobed (Acraspe- 

 dote Medusae, as 

 opposed to Craspe- 

 dote Medusae in 

 which a velum is 

 present). 



The marginal ten- 

 tacles are rarely absent 

 (tihizostoma) ; occasion- 

 ally there is only one 

 (Steenstrupia) or two 

 (Aeginopsis, Gemella- 

 rici) ; more frequently 

 there are four or some 

 multiple of four. Occa- 

 sionally there are six 

 or a multiple of six 

 (Carmarina, Fig. 101); or the tentacles may be numerous, in which case they 

 are either uniformly distributed round the edge of the bell ( Tiaropsis, Aurelia, 

 Fig. 100), or grouped in bundles (four bundles in ougainvillea, eight in 

 Lucernaridae, Fig. 134, and in Cyanea}. Occasionally the tentacles are removed 

 from the edge of the umbrella, and inserted either on the ex-umbrella (Narco- 

 medusae, Fig. 96), or on the sub-umbrella (Cyaneidac). The tentacles are 



FIG. 95. Sarsia mirabilis (from Chun, after Agassiz). A cras- 

 pedote ocellate medusa budded from Cory tie mirabilis. c.r 

 radial canal ; c.c circular canal ; ex.u ex-umbrella ; g inanu- 

 brium containing the elongated stomach and surrounded by 

 the gonads (manubrial gonads) ; o mouth ; t tentacles ; v 

 velum ; s.u sub- umbrella. 



