SEA-SQUIRTS. 571 



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 

 almost 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-salpae are produced 

 asexually by budding from a stolon within the body of the solitary form. In 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 off in each unit 

 of the chain. Simple salpae 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-salpae progress by an undulating, snake-like move- 



