360 THE SALPA. 



is usually the case, near the surface, those creatines present a gorgeous spectacle ; their vivid 

 phosphoric light being sufficient to illuminate, not only the 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 shij?. 

 They are occasionally collected together in incredible numbers. Ou two occasions, at mid- 

 night (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, wliich, 

 upon close examination, could be observed to proceed from myriads of luminous dots, occupy- 

 ing the situations of the small brown specks, noticeable in the fleshy structure of the mollusk. 

 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 phosph* »ric 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 Medusa?), its body being enveloped in a membrane that has no 

 luminous secretion. But when the mollusk is cut open in water, some of the brown specks 

 before mentioned will escape, and, difi'using 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." 



Oue last example of these remarkable mollusks is the Salpa, which is mentioned on 

 account of the curious phenomenon called "alternate generation."" wliich 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 Salpse are seen united in long chains, and swimming through 

 the ocean with a beautifully graceful movement that greatly resembles the undulations of a 

 swimming serpent. Sailors often call these chains of Salpa? 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 Salpa? 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 granddaughter, 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 Salpa\ each individual expands and contracts in exact unison, so that the force is 

 applied to the water in the strongesl 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 especially when pressed. 



