840 The Siphonophores. [ August, : , 
medusa is called Diplophysa mermis, and is regarded by many 
authors as the diphyizodid of this genus. 
There are several of the tubular meduse belonging to the 
Diphyz which depart very widely in general form from that as- 
sumed by Diphyes, although they are evidently more closely 
related to these than to the genus Agalma and its relatives. One 
of the most beautiful of these is a genus Praya,! of which several 
species have been described (Fig. 5). Praya has two nectocalyces 
at one extremity of a long, highly flexible axis or stem, which 
fit together side by side with the openings into their cavities 
facing outward and backward in the direction of the stem. Each 
bell is almost globular and has very flexible walls. In one spe- 
cies? one of the bells is much larger than the other, but in the 
others they are of about the same size. In many respects the 
swimming-bells of Praya are peculiar. One species (Fig. 5) has 
a somatocyst (ss) in both of the swimming-bells, while in others 
this structure is confined to one of the bells, as in other 
Diphyæ. The radial tubes in the species take a direct course 
from a common junction to the bell margin. 
The axis (s) is not capable of being drawn up into the interval 
between the bells, but as the animal swims in the water trails far 
behind and is thrown into many graceful curves by the on 
motion. This stem, when extended, is found to bear at intervals 
along its length little helmet-shaped transparent bodies (4) which 
in general shape resemble the primitive covering scale of the lar- 
val Agalma. The helmet and accompanying structures presently 
to be described, form the diphyizodids which in the subsequent 
growth of the Praya fall off one by one. After they have thus 
separated themselves from their attachments, they develop = 
medusz of very different outward shape from that of the atta 
young (Fig. 6). 
Although the helmet-shaped body (/) gives these diphyizooids 4 
their form, they are by no means the only structures in the clus- : 
ters. They cover other organs of a most important chm a 
Below each helmet-shaped body we find a small botryoidal “iY 
ter of bells bearing ova and probably spermatozoa, and a fl Eo 
shaped stomach (polypite) (e) which resembles a similar body 4 
1 Named from Porto Praya in the Cape Verde islands. 
* The species of the Praya found in the Gulf of Mexico 1s probably" 
Mus. Comp. Zoél., Vol. ey 
ew. Ball. : Ai q 
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bce Ser oo 
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