ACALEPHAE. 157 



Hydromedusae by their larger size and the greater thickness and 

 stiffness of their umbrella, the gelatinous tissue of which contains 

 a quantity of strong fibrillae, and a network of elastic fibres. 



Another characteristic of the group is derived from the stkrfcture 

 of the edge of the umbrella. This is divided by a regular number 

 of indentations usually into eight groups of lobes between which the 

 sense organs are contained in special pits (Fig. 130). 



The marginal lobes of the Acalepliae, like the continuous velum 

 of the Hydromedusae, appear to be secondary formations at the edge 

 of the disc. In the young stage known as Epliyra (Fig. 131), which 

 is common to most of the Ephyroninae, they are present as eight 

 pairs of relatively long tongue-like processes, and grow out from the 

 disc-like segments of the Strolila as marginal processes. An un- 

 divided marginal membrane (the vela- 

 rium), differing from the velum of the 

 Craspedota in containing prolongations 

 of the canals of the gastrovascular 

 system, is present in the Charyldeidae 

 alone. 



The Acalepliae differ from the Hy- 

 dromedusae in possessing, as a rule, 

 large oral tentacles at the free end of 

 the wide manubrium. These may be 

 regarded as being derived from an FlG 131 _ An Ephym seen from the 



unequal growth of the edges of the oral side. Ek marginal body ; Gf 



,1 r TI gastral filament; Re radial pouch 



mouth. They grow as four arm-like O f enteron ; o mouth, 

 processes of the manubrium from the 



angles of the mouth, and are placed perradially (see p. 118, note), 

 i.e., they alternate with the genital organs and gastric filaments. 

 In some cases the arms become forked at an early period, and four 

 pairs of arms are formed, the lobed tufted edges of which may again 

 divide and sub-divide into many branches. In this case, the margins 

 of the mouth and the opposed surfaces of each pair of arms fuse 

 in early life as described above, p. Ill (Rhizostomidae, Fig. 97). 



Further, there is to be observed in the Acalepliae two types of 

 structure, which we may term the type of the Scyphistoma and 

 the type of the Epliyra, respectively. To the former type, which 

 is called the Scyphomedusae, belong the sessile forms (Lucernaridae, 

 Fig. 134), and swimming forms which perhaps possess a direct 

 development (Tesseridae, Fig. 133, Charyldeidae, Fig. 136). The 

 edge of the umbrella is only incompletely divided into lobes ; marginal 



