TUNICATA. 373 



of sense, and almost cut off from all relation with the external world, 

 we can look for no very great developement of the nervous cen- 

 tres. There is one ganglion, however, lodged in the substance of 

 the mantle, distinctly recognizable, situate in the space between the 

 branchial and excretory openings, from which large nerves are given 

 off; but of other details connected with the nervous system of 

 the TUNICATA little has been made out. 



(409.) Many forms of Tunicated Mollusca are met with abun- 

 dantly in the seas of tropical latitudes, which, although allied to 

 Ascidians in the main points of their economy, present certain 

 peculiarities of structure that require brief notice in this place. 

 These, grouped by authors under the general name of Salpa, 

 are many of them so transparent that their presence in a quantity 

 of sea-water is not easily detected ; and their viscera, if coloured, 

 are readily distinguishable through their translucent integument, 

 which in texture seems to be intermediate between cartilage and 

 jelly. The body is oblong, and open at both extremities, the pos- 

 terior opening being very wide, and furnished with a crescentic valve 

 so disposed that water is freely drawn into the interior through 

 this aperture, but cannot again be expelled by the same channel ; 

 so that, being forced by the contractions of the body in powerful 

 gushes from the opposite end, it not only supplies the material for 

 respiration, but impels the delicate animal through the water in 

 a backward direction. The branchial chamber of Ascidia is con- 

 sequently in this case represented by a wide membranous canal, 

 which traverses the body from end to end ; but, instead of the 

 network of vessels lining the respiratory sac of Ascidians, a sin- 

 gular kind of branchial organ is placed 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 perpe- 

 tually renovated, and afterwards distributed, by a heart resembling 

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



The viscera, which occupy comparatively a very small space, 

 are lodged in a distinct compartment between the membranous 

 respiratory 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 re- 

 spiratory surface bring into it a sufficient supply of nutritive mole- 



