604 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
Both of the genera belonging there (Halobates and Halobatodes, with 
about a dozen species) are limited to the tropical and subtropical zone. 
The Challenger found them in the Atlantic between 35° north latitude 
and 20° south latitude; in the Pacific between 57° north latitude and 
23° south latitude. I myself observed Halobates 1umerously in the 
Indian Ocean, and on one day in crowds in the neighborhood of Belli- 
gam. Although they can dive, they never go into the depths. 
J.—TUNICATES OF THE PLANKTON. 
The tribe of mantle animals falls into two chief divisions, according 
to their mode of life. The ascidians belong to the benthos; all other 
tunicates to the plankton. The Copelata (or Appendicularide) are mor- 
phologieally the oldest branch of the stem, and are to be regarded as 
the nearest of the now living relatives of the Prochordinie, the hypo- 
thetical common ancestor of the tunicates and vertebrates (30, p. 605), 
The near relationship of the Copelata and the ascidian larva makes it 
very probable that the whole class_of ascidians has sprung from the 
primarily pelagic Copelata, and has diverged from this through the 
acquirement of a sessile mode of life. The Lucidiw or Pyrosomida, on 
the other hand, are probably secondarily pelagic animals, and sprang 
from the Celocormida, a benthonic synascidian group. The Thalidie 
(the Doliolide as well as the Salpide) are to be regarded as primarily 
pelagic animals. . These conditions are doubly interesting, because the 
tunicates in an exemplary manner demonstrate the peculiarities which 
the transition on one side to a sessile mode of life in the benthos (in 
case of the ascidians), and on the other to a free-swimming mode of 
life in the plankton (in the case of all other tunicates), has brought 
about. All the latter are transparent and luminous fragile animals, 
poor in genera and species, but rich in numbers of individuals. The 
ascidians, on the other hand, fastened to the bottom, in part littoral on 
the coast, in part abyssal in the deep sea, are much richer in genera 
and species, in many ways adapted to the manifold local conditions of 
the bottom, and mostly opaque. The few hyaline forms (e. g., Clavellina) 
may be regarded as the remnant of the old ascidian branch, which 
diverged from the pelagic Copelata. 
All planktonic tunicates are exquisite oceanic animals and all may 
appear in immense swarms of astonishing extent. Murray (6, pp. 170, 
521, 738, etc.) and Chierchia (8, pp. 32, 55, 75, ete.) met with great 
swarms of Appendicularia, Pyrosoma, Doliolum, and Salpa in the middle 
of the open ocean, both in the Atlantic and Pacific, particularly in the 
equatorial zone. I observed the same in the Indian Ocean, between 
Ceylon and Aden. Further, I have whole bottles full of closely pressed 
Thalidie, which Captain Rabbe collected in the middle of the Atlantic, 
Pacific, and Indian oceans, far removed from all coasts. In many log 
books also these swimming and luminous crowds of Salpa and Pyro- 
soma on the open sea, far from all coasts, are spoken of. On the other 
