442 THE SALPA. 



extent of ocean they occupy, but also the air above, rendering all surrounding objects 

 visible during the darkest night, and permitting a book to be read on the deck, or near 

 the stern cabin-windows of a ship. They are occasionally collected together in in- 

 credible numbers. On two occasions, at midnight (in lat. 20 and 40 N. Atlantic 

 Ocean), the ship sailed over many miles of water which they had illuminated, and in 

 which they were so densely crowded as to be taken in any amount by buckets or nets. 



When captured, they exhibited no signs of animation, and emitted a peculiar half- 

 fishy odor. When left in a vessel of sea-water, and allowed to be tranquil, their light 

 was withheld, or only sparingly displayed ; but when they were handled, or the water 

 in which they were contained was agitated, their body instantly became one blaze of 

 phosphoric light, which, upon close examination, could be observed to proceed from 

 myriads of luminous dots, occupying the situations of the small brown specks, notice- 

 able in the fleshy structure of the mollusc. Upon the irritating cause being removed, 

 the phosphoric light gradually expired, and the Pyrosoma remained in darkness until 

 again disturbed, when it once more illuminated objects with its vivid gleam ; and this 

 was repeated until after the death of the animal, when no luminous effect could be 

 produced. 



When living specimens were immersed in fresh water, they not only existed for some 

 hours, but emitted a constant light. Even after they had been so much enfeebled as 

 to cease to give light in sea-water, or after they had been seriously mutilated, their 

 phosphorescence invariably reappeared when they were put into fresh water, which 

 appears to act as a peculiar stimulus in reproducing the phosphoric light of these, as 

 well as of most other marine luminous animals. 



The Pyrosoma does not communicate its luminosity to water, nor to any object in 

 contact with it (like many luminous Medusas), its body being enveloped in a membrane 

 that has no luminous secretion. But when the mollusc is cut open in water, some of 

 the brown specks before mentioned will escape, and, diffusing themselves through the 

 fluid, shine independent of the animal : in this respect, as well as in their structure 

 and color, bearing some resemblance to the luminous scale on the abdomen of the small 

 fire-fly of Bengal." 



OUR last example of these remarkable molluscs is the SALPA, which is mentioned on 

 account of the curious phenomenon called " alternate generation," which is exhibited 

 by this creature. 



The Salpa takes two distinct forms, so entirely unlike each other that no one who 

 was unacquainted with the circumstance would imagine that they could possibly belong 

 to the same species. Sometimes the Salpae are seen united in long chains, and swim- 

 ming through the ocean with a beautifully graceful movement that greatly resembles 

 the undulations of a swimming serpent. Sailors often call these chains of Salpae by the 

 name of Sea Snakes. 



The remarkable characteristic in this creature is, however, that the solitary Salpa 

 produces a chain of united individuals, and that each of the united Salpae becomes the 

 parent of a solitary one. So that, as Mr. Rymer Jones happily remarks, " a Salpa 

 mother is not like its daughter or its own mother, but resembles its sister, its grand- 

 daughter, and its grandmother." When swimming at ease through the water, the 

 Salpa, like many other inhabitants of the ocean, is hardly perceptible, on account of 

 the extreme transparency of its structure, the only indication of its presence being a 

 kind of iridescence as the light plays upon the delicate membranes. The motive 

 power is obtained by regular contractions of the body, by which the refuse water is 

 rejected with some force, and thus drives the creature along by direct action, just as a 

 rocket is propelled through the air. It is a remarkable fact, that in the chain of united 

 Salpae, each individual expands and contracts in exact unison, so that the force is 

 applied to the water in the strongest possible manner. Sometimes the chains become 

 broken up, but the fragmentary portions do not seem to be at all inconvenienced by 

 the change in their condition, swimming about as actively as before. The creature is 

 very slightly luminous, giving forth its phosphorescent light when touched, and espe- 

 cially when pressed. 



