1876.) A Remarkable Life History and its Meaning. 643 
thizopods which swarm at the surface of the ocean after a long- 
continued calm, they are then met with in numbers which defy 
description, and cannot be conceived by those who have not act- 
ually seen them. 
Although single animals of our species are from one half to 
two thirds of an inch long, they are often so abundant that a 
bucket of water dipped at random from the surface of some 
| sheltered bay will be found to contain many hundreds or even - 
thousands. At such times collecting with the surface net be- 
comes impracticable ; for almost as soon as the net is placed in 
| the water it becomes choked with a solid mass of Salpx, so that 
| nothing more can enter it, and unless Salpx are what are wanted 
_ work must be abandoned until fresh winds again clear the water. 
| A drop from an organic infusion swarming with Paramecia, seen 
under a low power of the microscope will give some conception 
of the appearance of the ocean when Salpz are abundant, ex- 
cept that the water is not turbid like that of an infusion, but is 
perfectly fresh and clear; and no one who has not seen these 
animals under favorable conditions can form any conception of 
the amount of animal life which pure sea-water is able to sup- 
port. The various species of Salpa vary in size, from those less 
than half an inch to those which are nearly a foot long, our 
species, as already stated, being about two thirds of an inch in 
length. 
The animal (Figures 43 and 44) may be roughly described as 
4 barrel or hollow cylinder, b, with a valvular opening at each 
end. The valves which guard the anterior opening (Figures 43, 
45, and 55, e) are so arranged that while they allow the water to 
pass between them into the hollow chamber, A, they prevent it 
from passing out through the same opening ; while those at the 
Posterior opening (Figures 43, and 45, g) permit it to pass out but 
Prevent itsentrance. Around the barrel are a number of muscular 
belts like the hoops around a barrel (Figures 43, and 44, f ), the 
contraction of which diminishes the capacity of the hollow cham- 
ber, A, and thus drives the water out through the posterior open- 
i e animal forward. Upon 
a contraction. As this pumping action is constantly going on, 
w animal is continually moving forward in a nearly straight 
me, 
Wherever Salpa is found, two forms are met with, agreeing 
