360 THE SALPA. 
is usually the case, near the surface, these creatures 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 ship. 
They are occasionally collected together in incredible numbers. On 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, which, 
upon close examination, could be observed to proceed from myriads of luminous dots, oceupy- 
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 phosphoric light of these, as well as of most other marine luminous 
animals. 
‘* The Pyrosoma does not communicate its liminosity to water, nor to any object in contact 
with it (ike many luminous Medusve), 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, 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 mollusks 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 Salpze 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 Salpze 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 Salpz 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 Salpee, 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. 
