314 COELENTERATA SCYPHOZOA chap, 



at the base or between the base and the extremity, one or more 

 eyes. These organs differ from the statorhabs of the Hydrozoa 

 in having, usually, a cavity in the axial endoderm ; but as they 

 are undoubtedly specially modified marginal tentacles, they are 

 strictly homologous in the two classes. In nearly all the 

 Scyphozoa these organs are protected by a hood or fold formed 

 from the free margin of the umbrella, and this character, 

 although not of great morphological importance, serves to distin- 

 guish the common species from the Craspedote Medusae. It was 

 owing to this character that Forbes gave the name Steganoph- 

 THALMATA, or " covered-eyed Medusae," to the class. 



Another character of some importance is the presence in the 

 coelenteric cavity of all Scyphozoa of clusters or rows of delicate 

 filaments called the " phacellae." These filaments are covered with 

 a glandular epithelium, and are usually provided with numerous 

 nematocysts. They have a considerable resemblance to the 

 acontia of certain Anthozoa, and are probably mainly digestive 

 in function. These three characters, in addition to the very 

 important character of the position and method of discharge of 

 the sexual cells already referred to, justify the separation of the 

 Scyphozoa from the Medusae of the Hydrozoa as a distinct 

 class of Coelenterata. 



The umbrella of the Scyphozoa varies a good deal in shape. 

 It is usually flattened and disc-like (Discophora), but it may 

 be almost globular (Atorella), conical (some species of Periphylla), 

 or cubical (Cubomedusae). It is divided into an aboral and a 

 marginal region by a circular groove in the Coronata. The 

 margin may be almost entire, marked only by notches where the 

 statorhabs occur, or deeply lobed as in the Coronata and many 

 Discophora. Marginal tentacles are present in all but the 

 Ehizostomata, and may be few in number, four in Charyhdea, 

 eight in Ulmaris (Fig. 143), or very numerous in Awelia and 

 many others. The tentacles may be short (Aurelia), or very 

 long as in Chrysaora isosceles, in which they extend for a length 

 of twenty yards from the disc. 



The manubrium of the Scyphozoa is usually quadrangular in 

 section, and in those forms in which the shape is modified in the 

 adult Medusa the quadrangular shape can be recognised in the 

 earlier stages of development. The four angles of the manubrium 

 are of importance in descriptive anatomy, as the planes drawn 



