192 THREE CRUISES OF THE “BLAKE.” 
oceans, at the depth of about 2,000 fathoms. It is repre- 
sented by a single species, A. Wyvillei. In the Gulf Stream 
and North Atlantic we have two species of Atolla, discovered by 
the * Albatross.” They do not appear to be confined to deep 
water, but sometimes approach the surface. No discophore has 
as many sense segments as Atolla; and a marked feature of the 
oral surface of the bell 1s the large muscle found on the under 
Fig. 426. — Periphylla hyacinthina. 3. (Fewkes.) 
side of the corona. The ovaries of Atolla consist of eight kid- 
ney-shaped bodies arranged about a large and spacious stomach, 
which assumes the form of an inflated bag, opening into a recess 
in the walls of the corona, from which pers extend into the 
tentacles and sense-bodies. A. Bairdii is here figured. (Fig. 
427.) 
Some of the most interesting medusw discovered by the 
“Blake” belong to the Siphonophore. They are eminently 
pelagie in e own and wide-spread in their distribution. Pre- 
viously to the * Blake" expeditions we knew only a few genera 
of these beautiful animals from the American coasts. Although 
genera of siphonophores oceur in some of the most northern 
localities visited in Arctic exploration, the home of the group 
is essentially in the warmer waters. This group seems to be 
most varied and rich in the West Indian area. Before 1880, 
