Specimens vary in size from a few inches to upwards of four feet in length, and, as the name of the genus implies,[[22]] they are brilliantly phosphorescent. Sometimes they occur in innumerable multitudes, giving rise to a zone of greenish light extending for miles. Professor Moseley records that during the voyage of the Challenger in the North Atlantic a huge specimen of Pyrosoma spinosum, four feet in length, was captured. On tracing his name on its body, the word came out in letters of fire.
Fig. 20.
Section through wall of Pyrosoma, magnified, showing a single layer of ascidiozooids.
br, branchial; at, atrial orifice; tp, process of the test; br s, branchial sac.
(Herdman: Tunicata, Encyclopædia Britannica.)
In Pyrosoma elegans (exhibited), from Naples, the ascidiozooids are arranged in verticils, and the mouth of the cylinder is surrounded by a movable diaphragm; the outer end of each ascidiozooid is provided with a membranous spine. Six species of Pyrosoma are known. Pyrosoma atlanticum is found in the tropical Atlantic and Antarctic; P. giganteum in the Atlantic, Pacific, and Antarctic; and P. spinosum in the South Atlantic.
Order II.—Thaliacea.
The Thaliacea are free-swimming Tunicates, which exhibit alternation of generations in their life history. There are three families, Salpidæ, Octacnemidæ, and Doliolidæ.
Salpidæ.—The Salpas are transparent barrel-shaped organisms which occur in abundance at the ocean surface. They are so transparent that they are rarely seen, except in calm weather from the side of small boats; yet they frequently swarm in countless multitudes. From five to ten bands of muscles partially or entirely surround the body, like hoops. The branchial and atrial openings are at or near the opposite ends of the body. The branchial sac has almost disappeared, the dorsal lamina and ventral gutter (or endostyle) alone remaining, the interval between the two on each side representing an enormous stigma; the dorsal lamina, or “gill” is the transversely striated band passing obliquely across the body and forming the only barrier between the branchial and atrial cavities. Water enters at the mouth, and, by the contraction of the muscle-hoops, is driven out through the atrial aperture at the opposite end, which is then closed by a sphincter muscle. The elastic walls of the body expand, and water again enters through the mouth, the valve-like lips of which prevent its being driven out that way. The Salpa swims along in jerks, and along with each gulp of water takes in Radiolaria, Foraminifera, etc., which are retained by the mucus of the endostyle and carried to the gullet. The Salpa, in fact, lives, as Professor Brooks observes, in a “living broth,” so abundant is the food supply.
The intestines usually form an oval mass termed the “nucleus,” which is a conspicuous object at the posterior end.
Fig. 21.
Posterior part of solitary form of Salpa democratica-mucronata, showing a chain of embryos nearly ready to be set free.
gem, young chain of Salpæ; st, stolon; t, test; visc, visceral mass.