132 



THREE CRUISES OF THE " BLAKE 



oceans, at the depth of about 2,000 fathoms. It is repre- 

 sented by a single species, A. Wymllei. 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 is the large muscle found on the under 



Fig. 426. — Periphylla hyacinthina. j. (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 canals extend into the 

 tentacles and sense-bodies. A. Bairdii is here figured. (Fig. 

 427.) 



Some of the most interesting medusae discovered by the 

 " Blake " belong to the Siphonophorse. They are eminently 

 pelagic in character, 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 occur 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, 



