754 
ROBERT PAYNE BIGELOW 
Periphyllidae in a great group of Scyphomedusae, separate from 
the group containing the following genera, which lack the nodes. 
The genera Pelagia, Chrysaora, Dactylometra, Kuragea, and 
Sanderia constitute the family Pelagidae of Gegenbaur (Mayer). 
Pelagia is like Palephyra in having 8 tentacles and 16 mar- 
ginal lappets, but, as has been said, lacks the septal nodes. 
Chrysaora has 24 (3 X 8) tentacles, 3 between each successive 
pair of marginal sense-organs, and 32 marginal lappets. Dac- 
tylometra has 40 (5 X 8) tentacles, 5 between each pair of sense- 
organs, and 48 marginal lappets. Kuragea ends the series of 
progressive steps in complexity in this line with 56 (7 X 8) ten- 
tacles and 64 marginal lappets. Sanderia is an aberant form in 
which all the marginal parts of Pelagia have been doubled, thus 
having 16 marginal sense organs, and likewise 16 tentacles sep- 
rated by 32 left marginal lappets. 
Unfortunately nothing is known in regard to the structure of 
the sense-organs of Palephyra, and there is some doubt as to how 
far this genus should be regarded as representing an ancestral 
type of the group of Medusae classed by Haeckel in the order 
Discomedusae. 
The present account of the sense-organs deals with those of 
Pelagia cyanella Peron et Lesueur, the Chrysaora-like form of the 
Chesapeake Bay, and Dactylometra quinquecirrha L. Agassiz. 
First, three successive stages in the development of the sense-organ 
of Chrysaora will be described, leading to a description of the 
adult condition. This will be followed by a comparison of the 
adult sense-organs of Pelagia with the Pelagia-stage of Chrysaora 
and with the adult. Finally these structures in the adult Dacty- 
lometra and in some stages of its development will be compared 
with the corresponding stages in the other species. 
