458 



NATURAL SCIENCE. 



June, 



the body. The whole animal is surrounded by a thick, tough, trans- 

 parent " mantle." 



The food of Salpa consists of minute organisms like diatoms, 

 radiolarians, and so forth, and as these are practically unlimited in 

 quantity on the ocean surface where Salpa lives, Salpa are often found 

 swarming in numbers beyond description. They vary in size from a 

 quarter of an inch to eight inches. Though most abundant in the 

 tropics, they are found from further north than Scotland and Norway 

 to beyond Cape Horn and the most southerly parts of Australia. 

 " They are abundant," says Dr. Brooks, " only after the water has 

 been for some time undisturbed by winds : and as prolonged calms 

 are most frequent in warm seas, these 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 that nothing can enter it." 



Since the time of the poet and naturalist Chamisso, it has 

 been known that there are two generations in the life cycle of Salpa. 

 These are a generation of solitary individuals, and a generation of 



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Fig. 2. Fig. 3. Fig. 4. 



Fig. 2 — Chain of germ-cells surrounded by follicle-cells, lying in a blood canal. 



Fig. 3. — Section through a young embryo. 



Fig. 4. — Fertilised egg-cell surrounded by follicle-cells. 



individuals aggregated into chains or clusters. The specific characters 

 of the two generations differ from each other so widely, that without 

 rearing them there is no possibility of determining which simple and 

 solitary forms belong to each other. The solitary Salpa is born from 

 an egg which is carried within the body of the aggregated Salpa, and 

 the embryo, by means of a nutritive placenta, is nourished during its 

 development from the blood of the aggregated form. The aggregated 

 Salpa are produced asexually by budding from a stolon within the 

 body of the solitary form. The eggs arise at an exceedingly early 

 stage in the development of the animal. Thus the curious result 

 happens that three generations are simultaneously present in the 

 same animal. A solitary Salpa contains a developing chain of the 

 individuals of the aggregated form, and within these are already 

 present the eggs which are to give rise to the next generation of 

 solitary forms. Actually, the germ-cells can be seen in a solitary 

 form before the stolon is visible which is to give rise to the generation 

 of individuals in which the germ-cell will develop. As the stolon 

 forms, it includes within it the mass of germinal cells ; and as the 

 stolon elongates to form the chain of individuals, the mass of germ- 

 cells also elongates and becomes pulled out, so that a single egg-cell 

 is shut off in each individual of the aggregated series. This chain 

 of eggs is enclosed in a chain of follicle-cells, as in Fig. 2, which, 



