THE SCYPHOMEDUSAE 71 



DESCRIPTION OF THE MEDUSOID. The umbrella is generally 

 more or less flattened, and frequently exhibits externally a coronary 

 furrow which marks off the lappets near the edge of the bell. The 

 exumbrella is often variously marked by aggregations of pigment 

 cells and nematocysts. Throughout the group are recognisable in 

 connection with the edge of the bell, or just above it on the ex- 

 umbral surface, at least eight tentaculocysts and sixteen marginal 

 lappets, inherited from the Ephyra. 



The tentaculocysts (Figs. 10, 11) rarely exceed eight in number, 

 but twelve (Polyclonia) or even sixteen or thirty-two may occur. 

 They lie in incisions at the edge of the umbrella between two lappets, 

 which are, or are parts of, the guard lappets of the eight-rayed 

 Ephyra ; they are often protected on the exumbral aspect by the 

 development of a guard plate (Nausithoe). Each consists of a 

 short stalk, the base of the Ephyra tentacle, with a terminal endo- 

 dermal mass of crystalline otoliths, covered externally by ecto- 

 dermal sense cells with long sense hairs ; on the exumbral aspect 

 and proximal end of the stalk lies an ectodermal ocellus. Near 

 the base of the oral aspect of the stalk lies an ectodermal sense pit, 

 and a second sense pit is placed above the whole structure on the 

 exumbral surface of the guard plate. 



In addition to the sixteen marginal lappets of the Ephyra, 

 which lie at the sides of and protect the tentaculocysts, the filling 

 up of the eight adradial spaces between the eight primary Ephyra- 

 lobes results in the production of at least eight secondary marginal 

 lappets, which by fission and intercalation may be very largely 

 increased in number. 



The tentacles vary considerably in the different sub-orders. In 

 the Cannostomae they are short and solid ; in the Semostomae 

 they are long and hollow ; they are absent in the Rhizostomae. 

 They may be eight (Pelagia), twenty-four (Chrysaora), or even 

 more numerous (Cyanea). 



The subumbral cavity is generally shallow, and no true velum is 

 developed ; although the edge of the bell may in a few instances 

 form a thin velarium (Aurelia), it bears a different relation to the 

 nervous system, and is never inflected inwards. The subumbral 

 surface is in most cases perforated by the openings of the four 

 subgenital pits. These are chambers (Fig. 9) excavated in the 

 thickness of the subumbral wall, lined by ectoderm, and lying 

 interradially immediately under the generative organs, but not 

 communicating with the coelenteron ; they correspond to, and are 

 perhaps in some cases formed directly from, the subumbral funnels 

 of the Scyphistoma. In a few forms all four pits become confluent 

 centrally, the four openings persisting (Cannorhiza, Figs. 12, 13). 

 The manubrium is well developed, but assumes different forms in the 

 different sub-orders ; in Cannostomae it is a simple tube, crucial in 



