46 Bulletin Vanderbilt Marine Museum, Vol. VII 



a series of follicles in which the sex cells develop and then migrate 

 into a gelatinous lamella situated between the layers of entoderm. 



The egg is violet brown according to Mechnikov. Complete 

 and nearly equal segmentation occurs and a very large, central 

 segmentation cavity results. Invagination at the posterior end 

 of the body forms the gastrula ; the blastopore remains open, form- 

 ing the mouth of the larva. The mouth is of ectodermal origin and 

 forms by invagination of the posterior end of the larva but this 

 invaginated sac fills only a limited portion of the segmentation 

 cavity. The first pair of stomach pouches are entodermal and situ- 

 ated diametrically opposite each other. The second pair are ecto- 

 dermal and placed one each halfway between the first pair. The 

 entodermal pair develop two lateral pouches each; and later the 

 endodermal pair also develop two lateral pouches each ; thus the 

 larva has six ectodermal and six entodermal stomach pouches. 

 Next the ectodermal pouches develop four new adradial pouches, 

 giving the larva sixteen stomach pouches, ten of ectodermal and 

 six of entodermal origin. 



The external characters of the transformation of this free 

 swimming larva have been reported by Krohn, Kovalevski and 

 more recent writers. The larval mouth-end expands crater-like 

 with the mouth at the summit of this central zone. The subum- 

 brella develops from the depressed region around the cone. The 

 lappets grow out around the margin of the gastrovascular cavity. 

 At about this stage the larva loses the body covering of cilia and 

 then swims by rhythmical contraction of the oral disk, the free 

 swimming scyphostoma becoming an adult medusa without stro- 

 bilization. 



Technical description : Numerous excellent descriptions of 

 the adult medusa are available. The single specimen taken by the 

 '*Alva" off the Canary Islands is 20 millimeters diameter in the 

 preserved state, or about one-third the size of the average adult 

 specimens taken in the Mediterranean. The bell (preserved speci- 

 men) is subhemispherical with the apex flattish, the sides rela- 

 tively straight, sloping. The exumbrella has numerous verrucae, 

 arranged in irregular radiating series from the aboral center of 

 the exumbrella; these verrucae become less mammiform, more 

 oval and smaller toward the margin. Eight tubular, tapering 

 tentacles are present, each averaging a length equal to twice the 



