442 THE SALPA. 



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 incredible 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 he taken in any amount by buckets or nets. 



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

 odour. 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, noticeable 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 Medusae), 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 

 colour, 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 swimming 

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

 undulations of a swimming serpent. Sailors often call these chains of Salpse 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. Eymer 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 trans- 

 parency 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 Salpse, 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 especially when pressed. 



