THE SCYPHOMEDUSAE 63 



(taeniolcie) ; these are projections of mesogloea and endoderm from 

 the exumbral body wall towards the centre of the cavity. The 

 coelenteron, thus divided, extends into the adradial lappets of the 

 edge of the bell. In most forms the mesenteries, which have 

 a free edge in the more central parts of the organism, become 

 attached to the subumbral wall in the oral region, and are also 

 continued into the lappets ; they are, however, prevented from 

 reaching the extreme lip of the bell by a circular canal. In other 

 forms (Tessera) the mesenteries project but little from theexumbrellar 

 wall and have only a very short attachment to the subumbrella ; 

 the circular sinus is therefore very large. In many forms a pouch 

 of the ectoderm of the subumbrella, the interradial or suhumbral 

 fimnci, penetrates far into each mesentery. 



From the mesenteries grow the gastric filaments (phacellae) ; of 

 these there are four only, interradially placed (Tessera) ; or they 

 may be present in considerable numbers along both sides of each 

 mesentery (Haliclystus). In some cases the four mesenteries fuse 

 aboralh^ in the centre of the gastric cavity. 



A well-developed circtilar muscle runs round the edge of the bell in 

 all forms. Of the longitudinal muscles, the most marked are the eight 

 perradial and interradial bands, of which the latter lie immediately 

 under the ectoderm of the subumbral funnels, and are continued 

 deep into the substance of the mesogloea of the mesentery aborally. 



The sexes are separate. The generative organs are interradial, 

 and are horseshoe-shaped (Tessera), or are split by growth of the 

 mesenteries into bands at their sides (Haliclystus). 



Little is known of the reproduction of this group. The blastula 

 is apparently converted into the diblastula by a process intermediate 

 between delamination and true invagination. 



Order 2. Peromedusae. 



Definition. — Scyphomedusae with four interradial tentaculo- 

 cysts ; the tentacles are perradial and adradial in position. Four 

 mesenteries suspend the stomodaeum, and being attached to the 

 body wall at two points only, divide the peripheral coelenteron 

 into two large circular sinuses (confluent radial pouches). There is 

 no alternation of generations. 



The Peromedusae (Fig. 4) are medusiform, and bear a strong 

 resemblance to the Tesseridae among Stauromedusae. The bell is 

 conical and carries a well-developed manuhrimn ; no velum is present, 

 but a slight projection of the circular muscle subumbrally constitutes 

 the velarium. The edge of the bell has a complicated structure ; it 

 generally exhibits either four perradial tentacles, four tentaculo- 

 cysts on interradial lappets or pedalia, and eight adradial lappets 



