12 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



upwards with constantly diminishing area from the level of the suspen- 

 soria, Fig. 10 (if), to Fig. 6, which is taken very near the top of the bell. 

 Homologous structures exist in all the Scyphomedusse, and in some of 

 the LucernaridaB they are continued up even into the stalk of the 

 attached jelly-fish. 



(d) The Velarium. Charybdea, like most of the Cubomedusas, pos- 

 sesses a velum-like structure around the opening of the bell cavity 

 (Fig. 3, v). The velarium is a thin muscular diaphragm, resembling the 

 true velum in position and essential structures, but differing from the 

 velum in its origin, and in the possession of diverticula from the gastro- 

 vascular system, the velar canals. Of these there are in C. Xaymacana 

 very regularly sixteen, four in each quadrant. Their outline is seen in 

 Fig. 3 to be forked with small irregular accessory processes. As for its 

 origin, the velarium of the CubomedusaB is commonly accounted to have 

 arisen by fusion of marginal lobes, as in the case of the velarium of the 

 DiscomedusaB. Pending decisive ontological evidence, the slight notches 

 in the four perradii seen in Fig. 3 may perhaps be taken as slight indica- 

 tions of a primitive unfused condition, but the question will be brought 

 up again when the vascular lamellae are discussed. 



(e) The Frenula. Just as the stomach is attached to the walls of the 

 subumbrella in the four perradii by the suspensoria, so in the lower part 

 of the bell cavity the velarium is attached to the wall of the subumbrella 

 in the perradii by four structures similar to the suspensoria, the frenula 

 velarii. The frenula, like the suspensoria, resemble the brackets of a 

 shelf, with the difference that in the case of the frenula the bracket is 

 above the shelf, their purpose being evidently to keep the velarium stiff 

 against the outflow of water produced by the pulsations of the bell. 

 According to the greater need of strength in this case, we find the frenula 

 stouter, more buttress-like than the suspensoria. The gelatinous ridge 

 that gives them the necessary firmness is thickened so as to be triangular 

 in section, as shown in Fig. 16 (frn). 



(f) Musculature. As is general in medusaB, the muscular system, so 

 far as known, is restricted to the subumbrella. It has a very simple 

 arrangement, consisting of a continuous sheet of circular (/. e. horizon- 

 tal) striated fibres, which is interrupted only in the four perradii by the 

 radially directed muscle fibres of the suspensoria and the frenula. In 

 each quadrant, between the muscle of the suspensorium above and that 

 of the frenulum below, in an area just internal to the sensory niche, 

 there lies a space free from muscle. This interruption of the muscle 



