202 THE VOYAGE OF H.M.S. CHALLENGER. 



depressions arranged antero-posteriorly between the larger transverse vessels, but not 

 joinino- across them (PI. XXXIII. fig. 3). In this way a series of shallow pouches is 

 formed, opening off the peribranchial cavity. 



The dorsal lamina varies in its character, from a plain broad membrane with a smooth 

 edo-e and no markings, to a closely ribbed structure, with the free edge provided with one 

 or more series of large pointed knob or tooth-bke processes, the larger of which usually 

 correspond to the ends of the ribs, and are in the same line with the transverse vessels of 

 the sac. 



The tentacles are usually large, and very frequently of several sizes, arranged 

 symmetrically (PI. XXXIII. fig. 5). Each tentacle is long, tapering and filiform ; it is 

 triangular in cross section, and is placed so that one of the sides is anterior and the 

 opposite angle posterior. 



The dorsal tubercle is as a rule comparatively simple. The typical form in the genus 

 has a horse-shoe shape, with the aperture placed anteriorly, and the horns turned slightly 

 either inwards or outwards, but not coiled spirally, as in most of the Molgulidse and 

 Cynthiidae. 



The stomach and intestine lie upon the left side of the branchial sac, and the usual 

 arrangement is that shown in fig. 22, p. 201, where the ventrally directed oesophagus leads 

 into a globular stomach, which reaches to the ventral edge of the body. The intestine, 

 after emerging from the stomach, runs anteriorly, then dorsally, and then posteriorly 

 to form a loop (the intestinal loop) which is open posteriorly. It then turns dorsally and 

 anteriorly again, becoming the rectum which runs forwards near the dorsal edge of the 

 left side, and completes a. second loop (the rectal loop), which is open anteriorly. 



The branched ovary and testis occupy the intestinal loop (fig. 22, g.), and extend over 

 the adjoining parts of the stomach and intestine, while their ducts run along the pos- 

 terior and dorsal edge of the intestine and rectum, to open into the peribranchial cavity 

 anteriorly near the anus. 



A renal organ is present in the form of a mass of clear thin-walled and large vesicles, 

 usually containing concentrically laminated yellow and brown concretions. It occupies 

 the rectal loop and the adjoining regions of the left side of the body. 



Eleven species of Ascidia were collected during the Challenger expedition, and they 

 were all new to science, with the exception of the aj>parently common West Indian 

 species, Ascidia nigra, Savigny. 



Ascidia challengeri, n. sp. (PI. XXX.). 



External Abearance. — The shape of this species is irregularly oblong, with the 

 anterior end rather narrow ; the posterior, which forms the base of attachment, is slightly 

 broader, and the dorsal and ventral edges diverge slightly as they run backwards, and 

 are sometimes curved. The branchial aperture is anterior, terminal, or slightly on the 



