TUNICATA. 34)5 



oblique band across the interior; visceral nucleus posterior. Sexes com- 

 bined ; young produced by gemination in chains, consisting of individuals 

 unlike the parent and becoming oviparous, the alternate generations only 

 being alike. 



Distr. North sea, Brit. Medit. Australia, N. Zealand. 



The individual Salpiaus are from i an inch to 10 inches in length ; the 

 chains vary from a few inches to many feet, but are often broken up, indeed 

 the adults appear to be always separate. They swim with either end foremost, 

 although the pointed end would seem the normal one, as the motion is pro- 

 duced by the forcibly expulsion of the water from the mantle. Each orifice 

 is furnished with a valve, aud there is no division between the atrium and 

 respiratory cavity except the rudimentary gill, or " hypopharyngeal band." 

 The Salpa-chains also swim, with a regular serpentine movement. 



The solitary and aggregate forms differ so much that they were always 

 named and described as distinct species before the remarkable discovery made 

 by C/iamisso* that each form always produced the other. The free form of 

 S. democratica, Forsk. is a four-sided prism, with a rough surface, and 8 pro- 

 minent spines at the posterior' end; it has 7 muscular bands which comjdetely 

 encircle the body. The aggregate form {S. imccronata, Forsk.) is ovoid, 

 pointed behind, smooth, and has only 5 muscular bands, whose dorsal ends 

 are separate. 'Jluxley)\ 



The solitavj Salpae always contain a chain of embiyos winding spirally 

 round the visceral nucleus; the embryos are attached in pairs to a double 

 tube (or " proliferous stolon ") connected with the sinus to the right of the 

 heart. Sometimes they increase in size gradually from the heart outwards to 

 the free end of the stolon, but usually the embryos are developed iu groups, 

 and each portion of the series when it is detached consists of young 

 Salpasof the same size. These portions are liberated in succession through 

 an aperture produced in the tunic opposite the extremity of the stolon. 



The aggregate Salpae produce a single ovum at a time, which is attached 

 by a pedicle to the posterior part of the respiratory cavity. It remains there 

 until it has attained a considerable size, and exhibits the proliferous stolon 

 ah-eady partly developed, and those external characters which permanently 

 distinguish it from its parent. 



It was in Salfa that Hasselt first observed the periodic change in the 

 direction of tlie circulating currents. The heart itself is a muscular membrane 

 not forming a complete tube, but open on one side. The dorsal sinus con- 

 tains the long tubular filament (fig. 225, e) called the endostyle. In the 

 ventral sinus is the ganglion, and the auditory vesicle containing 4 otolithes. 

 The gill is a hollow column, or band, representing only the thoracic 

 vessel C' hypo-pharyngeal band ") of the Ascidians (fig. 226, d) and the respi- 



* Chiefly known in England as the author of P£ter Schlemthi. 

 t Phil. Trans, 1851, Part II. p. 567. 



