o80 



Sai.pid.e.- 



-MOLLUSCA.- 



-Salpid.e. 



were very iiidermite, presenting much of tlie appearance 

 of groat masses of fire, or rather of enormous red-hot 

 cannon balls; whilst those more distinctly seen near 



the surface perfectly resembled incandescent cylinders 

 of iron." The genus is represented by fig. 232 

 {Pyrosoma gitjanteurn) 



Order— MONOCHITONIDA, 



Inner sac adhering throughout to the external tunic. 

 Orifices without tentacular fringes. 



This order contains two families — one of which is 

 always free and floating; the other free also, but living 

 buried iii mud. 



Family I.-SALPIDiE. 



The Siilpm arc distinguished by their outer tunic 

 being thin, elastic, elongated, compressed, and open at 

 both extremilics, the two orifices being opposite eacli 

 other. They are free, and habitually swim on the 

 waters of the ocean, and are alternately solitary and 

 aggregated. The solitary individuals resemble short 

 but rather wide tubes, which are often of considerable 

 size, and so transparent tliat, as Professor Forbes says, 

 tliey look as if carved in crystal. 



The history of these Tunicaries is replete with interest, 

 especially on account of their singular mode of repro- 

 duction. Those occur under two conditions; at one 

 time appearing, as described above, in solitary indivi- 

 duals, floathig freely on the surface; at others, asso- 

 ciated into lengthened groups, or chains, composed of 

 numerous hidividuals of similar form, and swiramina 

 with regular serpentine movements through the tran- 

 rpiil waters. The individuals which compose these 

 chains contract and expand simultaneously, "keeping 

 time, as it were, like a regiment of soldiers on parade," 

 each chain tlius appearing to be a single being, acting 

 through the influence of a unique will. After floating 

 fur a certain time under this form the society is broken 

 up, and each individual, so separated from the chain, 

 propagates a young one like itself. This individual 

 grows to the size of its grand parent, and then brings 

 forth, in its turn, a social chain of young Salpa:, which, 

 again, give origin to the solitary individuals ; thus, as 

 Chamisso discovered, for it is to him we owe the know- 

 ledge of this curious fact, chained Salpa do not produce 

 chained Salpcc, but solitary Salpxc, ami these do not 

 reproduce solitary, but chained SalpiK; or, as this 

 naturalist expresses it, " a (Sn?;pa mother is not like its 

 daughter or its own mother, but resembles its sister, 

 its gi'auddanghter, and its gramlmother !" tlie alternate 

 r/<neralioiis only resembling each other. As the mem- 

 bers of one generation are very dissimilar from those of 

 the other, the species described previously to this dis- 

 covery of Chamisso's, were accordingly very much mul- 

 tiplied. The only conspicuous vital action in these 

 .Sc(/j;(e, says Professor Owen, are the rhythmical contrac- 

 tion and expansion of the mantle, in which the elasti- 

 cily of the outer tunic antagonizes the contraction of 

 the inner one. During expansion the sea-water enters 

 by the posterior aperture, and is expelled, in contraction, 

 by the anterior one; its exit by the opposite end being 

 prevented by a valve. The reaction of the jet, which 



is commonly forced out of a contracted tube, occasions 

 a retrograde movement of the animal. The currents 

 which successively traverse the interior of the animal, 

 renew the oxygenated medium upon the surface of 

 the respiratory organ, bring the nutrient molecules 

 within reach of the prehensile, subspiral, labial mem- 

 brane of the mouth, and expel the excrements and 

 generative products. Thus a single act of muscular 

 contraction is made subservient, by the admirable co- 

 adjustment of the dilferent organs, to the performance 

 of the functions of locomotion, nutrition, respiration, 

 excretion, and generation. The Salpce are found in 

 some of our northern seas, but tlioy are more abundant 

 in the l\Iediterranean, and in the warmer parts of the 

 ocean, in these localities they are remarkable for the 

 brilliant phosphorescent light they give out at night, 

 especially when the water is agitated. The long chains 

 of these animals swimming near the surface, have been 

 described as occasionally producing the effect of long 

 ribands of fire drawn along by the currents. 



Three genera have been enumerated. 



Genus Saepa. — Tliis genus is the typical and most 

 important one, and may be defined in the solitary 

 form, as oblong, subcylindrical, truncated in front by 

 the oral orifice, pointed posterior- 

 ly, anal orifice subterminal, each Fig. 233. 

 orifice furnished with a valve; test 

 thin, transparent; muscular man- 

 tle incomplete, forming a set of 

 transverse or oblique bands; man- 

 tle cavily lined by a system of 

 vascular sinuses; gill rudimen- 

 tary, consisting of two narrow, ob- 

 lique, unequal lamina;, forming an 

 oblique band across the interior; 

 visceral mass or imclcus situate 

 near the posterior extremity, and 

 very conspicuous, owing to the 

 brilliant orange, brown, or reddish 

 hues of the liver. The sexes arc 

 combined, and all the species yet 

 observed are viviparous ; the 

 young being produced by gem- 

 mation in chains consisting of salpa octofora 

 individuals uiJike the parent, 

 and becoming oviparous. The individual Salpa: vary 

 in length from half an inch to ten inches, and the chains 

 are found extending from a few inches to many feet. 

 A good many species have been described. The genus 

 is represented by fig. 233 {Salpa octofora). 



Genus Doliolum. — This genus was established for 

 some little animals nearly allied to the Salpa, which 

 are described as round, or in the form of a little cask, 

 transparent, open at the ends, and from two to ten 

 lines in length. The method of progression in these 



