120 



EMBRYOLOGY 



Pelagidffi and Cyanidse also the original character of the gastrovascular 

 system is jDreserved, the sixteen marginal (radial) pouches being retained 

 as broad spaces separated by only narrow concrescence-bands, and not 

 communicating by any circular sinus. More complicated conditions are 

 found in the Aureliidas, the metamorphosis of which froin the Ephyra 

 has been accurately described by Claus (No. 102 and jS'o. 3) for Aurelia 

 and Discomedusa (Umbrosa). In Aurelia the enlargement of the disc 

 is accompanied by the development of eight intermediate (adradial) 

 marginal lobes, on the ex-umbrellar surface of which numerous short 

 tentacles, arranged in a single series, are developed (Fig. Gl i). While 



Fig. 51.— Development of the margin of the disc anl the canal system of 

 Aurelia ciurita (after Claus). A, quadrant of an Bphyra disc 3 mm. in breadth ; 

 B, quadrant of a young Aurelia with a disc 9 mm. in di.ameter; i, intermediate 

 (adradial) marginal lobes; o, ocellar lobes; s/c, sensory bodies; t, tentacles (some- 

 what removed to the ex-umbrellar side). 



the disc thus gradually enlarges, the sixteen marginal pouches grow out 

 into elongated, narrow radial vessels, between which the concrescence- 

 bands extend as broad areas of the vascular plate. By the separation in 

 places of the two lamellae of this plate, secondary canals are developed, 

 by means of which there is formed first a zigzag, and subsequently a 

 peripheral circular communication between the different radial vessels 

 {rin;/ siniis\, besides numerous lateral branches of the radial vessels (Fig. 

 (51). The four oral arms, beset with papillre, arise as outgrowths from 

 the four angles of the mouth. That which especially interests us in the 

 metamori^hosis of the Ephyra of Rhizostoma, made known by Claus 

 (Nos. 3 and 103), is the metamorphosis of the oral stalk (manubrium). 

 The broadened ends of the four oral arms grow out into bifurcate lobes, 

 thus producing the fundaments of the eight oral arms, while by a 

 similar i)rocess of growth at the ends of these the fundaments of the 



