1230 



TUNICATA. 



any fluid or solid in contact with it. But if 

 the Pyrosoma were cut open and immersed 

 in water, the brown particles that escaped 

 diffused themselves through the fluid, and 

 shone as numerous scintillations, independent 

 of the perfect structure. General friction or 

 contact was not essential to elicit the perfect 

 light of the Pyrosome, since touching one 

 small portion of the body was sufficient to 

 produce a brilliant glow throughout the 

 whole. 



Mr. Bennett at the same time made the 

 following observations on the effect of fresh 

 water on phosphorescent marine animals. 

 " Fresh water appears to act as a powerful 

 and permanent stimulus on marine Noclilucce. 

 Those that have intervals of repose from 

 their phosphorescence immediately emit light 

 when brought into contact with fresh water ; 

 and this fact was strikingly exhibited in the 

 Pyrosomata* When placed in a vessel of sea- 

 water and permitted to remain quiet, these 

 Molluscs afforded no light, and, when touched, 

 gleamed forth only as long as the irritating 

 cause remained, and then gradually returned 

 to their original state. When, however, the 

 same creatures were placed in a vessel of fresh 

 water, they never ceased glowing with their 

 brightest refulgence until life was extinct, 

 which was not until the lapse of several hours. 

 When also the same Molluscs were mutilated, 

 or so near death as to refuse to emit light 

 upon irritation in sea-water, immersing them 

 in fresh water produced at least a temporary 

 revival of their brightest gleam." 



ANATOMY OF SALP^E. The subcartilagi- 

 nous test of Salpcs is more or less cylindrical 

 and flexible, and when taken out of the sea, 

 often collapses into an amorphous mass. Float- 

 ing in the water, it is iridescent or opaline, 

 reflecting the sun's rays in beautiful rainbow 

 tints, and hence has been derived its name of 

 " sun-fish." The test and its lining membrane 

 are so diaphanous, that the whole structure of 

 the animal can be seen through them. The 

 exterior of the test is generally smooth, but 

 sometimes bears minute shiny protuberances of 

 a tissue similar to its own, as in S. Tilesii. The 

 integument over the viscera is thicker and 

 often of a firmer consistence than the rest of 

 the test. Its form varies considerably, not only 

 in different species, but in the conditions of 

 isolation and aggregation, in which each species 

 alternately exists, and also in the different 

 stages of growth of the individual, especially 

 in an associated state. 



The test is usually either oval or oblong, 

 but various terminal and lateral processes 

 considerably affect its general contour. Each 

 of its extremities is open by an orifice ; and, 

 in the aggregated individuals, it is perfo- 

 rated by other much smaller apertures at 

 the points of contact between the neighbour- 

 ing individuals of a group. 



The internal surface of the test is lined by 

 the mantle, a thin, toughish membrane, which 

 is often more conspicuous than the highly 

 transparent external envelope. The mantle 

 is furnished with large, generally transverse, 



muscular bands, the arrangement of which 

 differs according to the species and according 

 to the separate or associated state of existence. 

 Its inner surface is lined with a soft mucous 

 coat of fine epithelial tissue. The mantle is 

 more or less closely attached to the inner sur- 

 face of the test, especially at the two orifices 

 (being, indeed, continuous with the test at 

 these points), at the protuberances that arise 

 from the test, and along the inner surfaces of 

 the longitudinal furrows that sometimes tra- 

 verse its external surface, as in S. cordiformis, 

 where, on the superior and inferior surfaces, 

 the test is depressed into broad, deep sulci. 

 In specimens preserved in spirit, and even in 

 some recent specimens, this membrane will 

 sometimes separate itself, together with the 

 viscera and vessels, from the outer tunic, and 

 fall through one of the external orifices. 

 Salpce have been met with floating in the sea, 

 and executing, in some degree, their usual 

 movements of contraction and dilatation; in 

 which, from mutilations caused by parasites 

 or other accidents, little remained except a 

 few muscular bands of the mantle. 



Considerable confusion has existed among 

 naturalists as to which is the anterior extre- 

 mity, and which the superior surface of the 

 Salpa. We shall regard, as the anterior or 

 branchial orifice, that in the neighbourhood of 

 which the stomach and heart are usually found 

 (fig. 787.); and as the posterior or anal orifice, 

 that which points in the direction of the ani- 

 mal's retrograde motion ; and as the superior 

 or dorsal surface, that on which the nerve- 

 ganglion is present; and as the inferior or 

 ventral, that in close contact with which the 

 viscera and heart, forming the i( nucleus " or 

 "paquet " of authors, are placed. 



The orifices are either terminal, or are situ- 

 ated at the base of the terminal prolongations 

 of the test. The anterior orifice is destitute 

 of tentacular appendages ; it is furnished with 

 sphincter muscles ; it is simple and tubular 

 in S. cristata, S. Tilesii, and S. scutigera, &c., 

 and transverse in S. cordiformis, S. zonaria, 

 &c. When the dilatation of the animal is 

 drawing water into the cavity of the body, the 

 sphincters of this orifice are brought into action 

 and close it, so that the water enters by the 

 opposite extremity. 



The posterior orifice is a transverse slit, 

 furnished with a few muscular fascicles, and 

 is larger than the opposite orifice. In S. cris- 

 tata its superior lip is simple and thin; its 

 inferior lip is externally a rounded ridge, 

 formed by a fold of the test, which, within its 

 inner border, constitutes a semilunar valve. 

 This allows the water to enter easily, but pre- 

 vents its escape when the animal contracts 

 itself. 



The constrictor muscles of the mantle are 

 often subannular, interrupted on one or the 

 other surface, decussating in some, connected 

 in others. The following are a few examples 

 of the disposition of these muscular bands. 

 In S. cristata, around the tubular anterior ori- 

 fice, are 4 5 sphincter muscles ; the first of 

 which, as well as the last two, are continuous 



