SEA-SQUIRTS. 
57i 
gill-bars abut. The only part of the salpa that is not transparent is the thick 
mass of viscera (e, c ) at the hinder end of the body; while the muscular bands, 
by the contraction of which the water is driven through the barrel, may be 
compared to the hoops of the latter. Externally the whole animal is invested 
with a thick, tough, transparent tunic; and in some forms there are two tail-like 
appendages to the hinder end of the body. Such is the structure of a salpa; but 
there are two generations in the life of these creatures, namely, the simple form, 
and the chain-like or aggregate form ; the first being shown in the upper, and the 
second in the lower figure of the Plate facing p. 561. It will be observed that 
in the chain the individuals are attached to one another by their upper and lower 
surfaces, and thus have these two apertures free; and when taken from the water 
the whole chain, which is several feet in length, can be easily resolved into its 
component units. The specimen represented in the annexed figure is one of these 
detached units from a chain, the projection marked g being for the purpose of 
attachment to the neighbouring individual. Although extremely interesting and 
curious, the whole history of the development of salpse is so complicated that it is 
alfnost impossible to explain it fully in a popular work. It may be stated, 
however, that the solitary salpa is born from an egg carried within the body of 
one unit of the aggregate form, the embryo being nourished by means of a placenta 
from the blood of the parent. On the other hand, the chain-salpse are produced 
asexually by budding from a stolon within the body of the solitary form. I 11 the 
chain-salpa the eggs arise, however, at an exceedingly early period of its develop¬ 
ment, with the curious result that three generations are present at one time in a 
single individual. Thus a solitary salpa has within it the buds of an aggregate 
salpa, the units of which may each contain eggs which will ultimately develop 
into the next solitary form. And, as a matter of fact, in a solitary salpa the 
germ-cells of the embryo of the next solitary form are actually visible before the 
development of the stolon which is to give rise to the chain-form. As the stolon 
forms in the body of the latter, it includes within it the mass of germinal cells; 
and while the former elongates to form the chain of units, the mass of germ-cells 
likewise lengthens, with the result that a single egg-cell is shut otf in each unit 
of the chain. Simple salpse vary in size from a quarter of an inch to upwards of 
eight inches; and in some parts of the ocean-surface are met with in incalculable 
swarms. Although more abundant in tropical than in the cooler seas, their 
northward range extends beyond Scotland and Norway, while to the south they 
have been taken below the latitude of Cape Horn and the most southern point of 
Australia. Dr. Brooks writes that “ they are abundant only after the water has 
been for some time undisturbed by winds; and as prolonged calms are most 
frequent in warm seas, those waters are most favourable for the development of 
these animals, which multiply with most astonishing rapidity. The smaller species 
are often so abundant that for hundreds of miles any bucketful of water dipped up 
at random, will be found to contain hundreds of them. In such places collecting 
with the surface-net becomes impracticable, for almost as soon as the net is dropped 
into the water, it becomes choked with a mass so dense that nothing can enter it.” 
The food of these creatures consists of minute marine organisms, both animal and 
vegetable. In swimming, chain-salpse progress by an undulating, snake-like move- 
