iv PHYLUM CCELENTERATA 175 



It has been shown that, under exceptional circumstances, the 

 egg of Aurelia develops into scyphulse which do not undergo 

 transverse division, the entire scyplmla becoming metamorphosed 

 into a single adult. 



2. GENERAL STRUCTURE AND CLASSIFICATION. 



The Scyphozoa may be defined as medusoid Ccelenterata, having 

 the same general structure and arrangement of the layers as the 

 medusoid Hydrozoa, but differing from them in the possession of 

 endodermal gastric tentacles ; in having gonads the sexual cells 

 of which are lodged in the endoderm and which discharge their 

 products into the digestive cavity ; in the absence of a true velum, 

 and, in nearly all cases, in the presence of sense-organs in the form 

 of hollow sense-clubs or ' tentaculocysts. Whether a stomodseum 

 or ectodermal gullet occurs is uncertain. As in the Hydrozoa, the 

 medusa develops directly from the egg in some Scyphozoa, while 

 in others there is a sort of alternation of generations, a polype- 

 form (agamobium) giving rise to the medusa-form (gamobium) by 

 a process of transverse fission. In the majority, however, nothing 

 is known of the life-history, the process of development having 

 been worked out only in a few cases. 



As far as is known, the segmenting embryo gives rise to a gastrula 

 by invagination in all with the exception of Lucernaria and its 

 allies : by the partial or complete closure of the blastopore a 

 planula is produced, at one end of which a second invagination 

 takes place, forming the stomodseum. 



The Scyphozoa are divisible into four orders, as follows : 



ORDER 1. STAUROMEDUSJE (LUCERNARIDA). 



Scyphozoa having a conical or vase-shaped umbrella, sometimes 

 attached to external objects by an ex-umbrellar peduncle : no 



tentaculocysts. 



ORDER 2. CORONATA. 



Scyphozoa having the umbrella divided by a horizontal coronary 

 groove : four to sixteen tentaculocysts. 



ORDER 3. CUBOMEDUS^E. 



Scyphozoa with a four-sided cup-shaped umbrella : four per- 

 radial tentaculocysts. 



ORDER 4. DISCO-MEDUSA. 



Scyphozoa with a flattened saucer- or disc-shaped umbrella : 

 not fewer than eight tentaculocysts four per- and four inter- 

 radial. 



