HYDROMEDUSAE 303 



in Willia stellata has recently been described by Ranson (1937, pp. 321-4, fig. i) who 

 found that they are not situated externally upon the exumbrella, but immediately below 

 the ectodermal epithelium of the exumbrella and connected with the umbrella margin 

 by a narrow string of endoderm cells. These structures are called by Ranson "cnido- 

 thylacies". 



The stomach is short, about one quarter the length of the umbrella cavity, and has a 

 number of lateral lobes; the number of lobes is variable, but apparently the normal 

 number in adult specimens is eight. The mouth is very variable in shape, and in many 

 specimens the shape is probably due to contraction and contortion. In the early stages 

 it is circular when fully expanded, but it has large lips, usually four, when closed. 

 Later on, definite lobes or folds begin to appear (Plate XVII, fig. 11), and in the large 

 adults the lobes or folds are numerous and closely packed together. Sections show that 

 the lobe or fold is tricornuate. The true form of the mouth in the adult cannot be 

 ascertained, beyond that the margin is closely folded ; none of the specimens have the 

 mouth properly expanded, in all it is more or less contracted. 



The gonads surround the stomach and also the lobes of the stomach. Several series 

 of sections were cut for the purpose of seeing the exact position of the gonads (Plate 

 XVII, fig. 12). 



The length of the tentacles, when fully extended, is about i\-z times the diameter of 

 the umbrella. The basal bulb contains a central mass of dark brown or black pigment 

 when fully developed (Plate XIV, fig. 9), but at an early stage the bulb is yellowish brown. 

 The basal bulb is inside the wall of the umbrella, and the proximal part of older tentacles 

 is partly embedded in the jelly of the umbrella margin, the point of issue of the free 

 tentacle being somewhat displaced towards the exumbrella. In some other species of 

 Williidae the tentacles are known to issue from the umbrella in a similar way, and in this 

 respect, as well as in the structure of the gonads, the Williidae resemble certain species of 

 Moerisiidae and Olindiidae. The structure of the tentacle itself is likewise in accordance 

 with that observed in these families of Limnomedusae ; the tentacle is hollow, though in 

 the greater part of its length the central lumen is very narrow. In the proximal part of the 

 tentacle the nematocysts form minute, scattered groups ; at a short distance from the base 

 the groups of nematocysts are larger and broader and are arranged in two longitudinal 

 rows (Plate XVII, fig. 12); still farther out these two rows become united, forming 

 a series of crescent-shaped clusters on the abaxial side of the tentacle, and towards the 

 distal end of the tentacle the clusters gradually become annular, surrounding the tentacle 

 as complete, distinctly separated rings. In Willia stellata and in several species of 

 Proboscidactyla the tentacles are likewise known to have annular clusters of nematocysts. 



Sections show that a ring canal of the usual form is absent. The radial canals have 

 a distinct lumen and open into a large cavity inside the basal bulb of each tentacle, but 

 no open canal is seen round the margin of the umbrella. There is a band of endoderm 

 cells round the margin, in the position of the ring canal, but no trace of a cavity inside 

 the cells, exactly as in other species of Willia and Proboscidactyla. The band of cells 

 round the margin gives the external appearance of a ring canal. 



