256 NATURAL HISTORY. 



The characters and distribution of the remaining genera may be succinctly stated. 



Genus Distomus. Covering semi-cartilaginous, fixed, variable in lorm, groups numerous, 

 generally circular, orifices six-rayed. Two species are found in Europe, Africa, and Australia. 



Genus Diazona. Common covering gelatinous, fixed, sometimes stalked, groups prominent, 

 ranged in circles on a concentric disc, like the petals of a flower, with the atrial cavity in the 

 centre ; the branchial orifice is six-rayed. One species occurs in the Mediterranean. 



Genus Polyclinum.* Covering gelatinous or cartilaginous, variable in form, systems numerous, 

 convex, somewhat stellate, tunicaries, groups of individuals ten to one hundred and fifty, at unequal 

 distances. Seven species are known in Britain, the Red Sea, India, &c. 



Genus Aplydium. Systems numerous, prominent, annular or sub-elliptical, tunicaries three to 

 twenty-five in single rows, equidistant from the centres, branchial orifice six-rayed. Living attached 

 to stones, &c., in deep water. They are found in the Red Sea and Europe. 



Genus Sidnyum. The animals of Sidnyum partake of the characters of Syncecium and 

 Aplydium, resembling the former in the structure of their stomach, and the latter in their branchial 

 apparatus. Each has an eight-toothed branchial orifice, and a simple tubular vent folded against the 

 thorax. The ovary is pedunculated and very conspicuous at the extremity of the animal. (Adams.) 



Genus Syncecium. Semi-cartilaginous, cylindrical, stalked, solitary or gregarious, systems circular, 

 terminal tunicaries six to nine in a group ; apertures six-rayed. Only one species is known, obtained 

 from Spitzbergen. 



Genus Siyillina. Covering solid, gelatinous, conical, elongated, erect on a stalk, individuals in 

 irregular circles one above another, openings six-rayed. A single species is found living in tropical seas. 



FAMILY IV. PYROSOMID^. 

 The animals are compound, free and pelagic. 



Genus Pyrosoma.} The body is cylindrical, hollow, non-contractile, cartilaginous, open at one end 



only, and covered externally by the numerous pointed zooids, 

 arranged in whorls; the interior is mammillated and pierced by the 

 excurrent orifices of the tunicaries. The Pyrosomes are from two to 

 fourteen inches long, and from half an inch to three inches in cir- 

 PYROSOMA. cumference. They are made up of innumerable individuals united 



A, the atnai or excurrent opening. ^ Q by side. The inhalent openings are external, the exhalent 



within the tube, and the result of so many little currents discharged into the cavity is to produce one 

 general outflow, which impels the floating cylinder, with its closed end forward, through the water. 



June 15th, 1850, lat. 45 S., long. 110 W. : " The sky was clear but moonless, and the sea calm, 

 and a more beautiful sight can hardly be imagined than that presented from the decks of the ship as she 

 drifted, hour after hour, through this shoal of miniature pillars of fire gleaming out of the dark sea, 

 with an ever-waning, ever-brightening, soft bluish light, as far as the eye could reach on every side. 

 The Pyrosoma floated deep, and it was with difficulty that some were procured for examination 

 and placed in a bucketful of water. The phosphorescence was intermittent, periods of darkness alternating 

 with periods of brilliancy. The light commenced in one spot, apparently on the body of the zooid, 

 and gradually spread from this to the centre in all directions ; then the whole was lighted up. It 

 remained brilliant for a few seconds, and then gradually faded and died away, until the whole mass was 

 dark again." (Huxley, Philosophical Transactions, Part II., 1851, p. 580.) 



M. Peron first observed the phosphorescence of the Pyrosomes in a squall at sea. He says : 

 " Suddenly we discovered at some distance a great phosphorescent band stretched across the waves and 

 occupying an immense tract in advance of the ship. Soon we reached the illuminated tract, and 

 perceived that the prodigious brightness was certainly and only attributable to the presence of an 

 innumerable multitude of animals floating on the waves. Those seen near the surface of the 

 water perfectly resembled small incandescent cylinders of iron." 



FAMILY V. SA 



The animals are free, oceanic, alternately solid, and united in circular or lengthened groups. These 

 Salpa chains vary in length from a few inches to many feet, and swim through the water with a 

 * Greek, polys, many ; Mine, a couch. t Greek, pin; fire ; soma, a body. 



