488 TUNICATA. 



within it. This consists of a long vascular riband attached by both its 

 extremities to the walls of the canal, through which the water rushes ; 

 and of course, being freely exposed to the influence of the surrounding 

 medium, the blood contained in this curious branchial apparatus is per- 

 petually renovated, and afterwards distributed, by a heart resembling 

 that met with in the genus last described, to all parts of the body. 



(1281.) The viscera, which occupy comparatively a very small space, 

 are lodged in a distinct compartment between the membranous respira- 

 tory channel and the external gelatinous investment, or soft shell, as we 

 might properly term it. The mouth is a simple aperture, situated near 

 the upper extremity of the branchial organ ; and probably, as in Ascidia, 

 ciliary currents rushing over the respiratory surface bring into it a 

 sufficient supply of nutritive molecules. The stomach is capacious, and 

 covered with parallel rows of large white filaments, that seemingly 

 represent the liver; and -the alimentary canal, which is perfectly 

 simple, runs to the posterior extremity of the animal, and terminates 

 there by a wide opening*. Two oblong bodies, each consisting of a 

 granular substance, are seen upon the ventral surface of the body, lodged 

 between the external and internal membranes ; these, no doubt, are the 

 ovaria, and form a reproductive system as devoid of complication as that 

 of the sessile Ascidians. 



(1282.) A very remarkable feature in the history of these animals is 

 that many species are found swimming together in long chains, appa- 

 rently adhering to each other by little . suckers, but without organic 

 connexion ; and, what is still more strange, it would appear, from the 

 observations of M. de Chamissof, that such aggregated animals give 

 birth to insulated individuals of very different appearance, which in 

 their turn reproduce concatenated forms resembling their progenitors ; 

 so that the alternate generations are quite dissimilar both in conforma- 

 tion and habits. 



(1283.) The observations of Chamisso have in later times been 

 substantiated and carried out by the researches of Krohn, Steenstrup, 

 Eschricht, Milne- Ed wards, and others ; and the phenomena connected 

 with the process are so interesting that it will be necessary to lay before 

 the reader a brief abstract of the result of their labours. 



(1284.) TheJSalpce are all viviparous, and each species is propagated 

 by an alternate succession of generations most dissimilar from each 

 other in their forms, habits, and mode of increase. The concatenated 

 Salpse produce but a single egg apiece, which is distinctly visible in 

 the interior of their transparent bodies ; they seem, moreover, to be 

 bisexual, having, as it would appear, two generative functions to per- 



* For excellent drawings representing the anatomy of various Salpce, the reader 

 is referred to the Descriptive and Illustrated Catalogue of the Physiological Series 

 of Comparative Anatomy contained in the Museum of theKoyal College of Surgeons 

 of England, vol. i. plates 6 & 7. t Dissert, de Salpa, Berlin, 1830. 



