W. K. BROOKS ON THE GENUS SALPA. 5 



floating animal through the water in the opposite direction. Around the 

 mouth the muscles are so placed that they close the lips and prevent the 

 water from escaping in this direction when the barrel contracts. 



The body is inclosed in a thick outer mantle, Plates I and II, which 

 by its elasticity antagonizes the muscles, and expands the barrel and 

 draws in a fresh mouthful of water. In most species the atrial aperture, 

 Plate I, Fig. 3, is encircled by sphincter muscles which constrict it and 

 prevent the water from entering during expansion, so that the animal 

 moves forwards, by jerks, along a column of water which passes through 

 its body. 



The food of salpa consists of radiolarians, diatomes, and other micro- 

 organisms which float in the water, and as these lodge on the inner sur- 

 face of the barrel they are gathered up and swept through the oesophagus 

 into the stomach in a way which will be described soon. The supply of 

 this food is unlimited, and salpae are often found swarming at the sur- 

 face of the ocean in number beyond description. 



In size they range from 6 mm. or about one-fourth of an inch, the 

 average length of the aggregated form of Salpa democratica, Plate II, 

 Fig. 1, to about 20 cm. or eight inches, the length of a large specimen 

 of the solitary form of Salpa costata, Plate IV, Fig. 4. 



Although they are met with in the greatest variety and abundance 

 in the warmer parts of the ocean, they are by no means confined to the 

 tropics, and they have been found in great numbers north of Norway 

 and Scotland and south of Cape Horn and the most southern points of 

 Australia and New Zealand. 



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, these waters are most favorable for the development of these 

 animals, which multiply with 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 of salpse so that nothing more can enter it. 



A drop from an organic infusion swarming with infusoria, seen 

 under a low power of the microscope, bears some resemblance to the 

 surface of the ocean when salpae are abundant, except that the water 

 is not turbid like the infusion, but beautifully clear and transparent. 



