IflS 



THE VOYAGE OF H.M.S. CHALLENGER. 



Ascidia is the largest and best known genus of the Ascidiidaa, and may be considered 

 as the typical form. In shape it is usually irregularly ovate or elongated antero-posteri- 

 orly, and it is attached by the posterior end or part of the left side of the body. It is 

 very rarely 1 covered with incrusting sand and other foreign bodies. Usually the bran- 

 chial aperture is surrounded by eight lobes and the atrial by six, but the former may 

 have seven or nine, and the latter five or seven. 



The test is usually thick, but soft and flexible, and more or less transparent. In a 

 typical species (e.g., Ascidia mentula, 0. F. Muller) it is plentifully supplied with blood- 

 vessels, and the terminal knobs in the outer layers are surrounded by quantities of large 

 ovate bladder cells (fig. 18, a). 



^fTt- 



<$ 



a/. 





I© / 



.?*■■& 



v-- * ;#-\«- 



Fig. 18.— Transverse section through the test of Ascidia, showing the matrix in which lie large bladder cells (c) scattered in the inner 

 layers, and smaller bladder cells (a) near the surface (the left side of the figure), blood-vessels (4) with terminal knobs, and pigment 

 cells (d) — magnified about 40 times. 



In some species (e.g., Ascidia nigra, Savigny) many of the cells become pigmented, 

 thus colouring the whole test and rendering it opaque. In a few cases the test is very 

 thin and membranous, and appears to have no vessels. 



The mantle is never very thick. It is usually fairly muscular on the right side where 

 it lies over the branchial sac, but the muscle bands are not arranged according to a 

 definite system, as they are in many of the Cyntliiidse. There are a number of bundles 

 which radiate from the bases of the siphons, and some others which cross the anterior and 

 dorsal region lying between the branchial and atrial apertures. These usually form longi- 

 tudinally running sets of bundles, which, however, branch and anastomose so as to make 

 an irregular network. They are crossed at right angles, and obliquely, by a series of more 

 or less transversely running bands, which terminate at the dorsal and ventral edges, or 

 very slightly beyond them, as the left side of the body over the viscera is almost or 

 entirely destitute of muscular fibres. The siphons are very rarely prominent, 3 and the 

 sphincters are usually only of moderate strength. 



1 Ascidia involute, Heller, is one of the exceptions. 



2 Ascidia longitubis, Traustedt, has both siphons enormously elongated, and Ascidia pyriformis, Herdinan (PI. txxiv. 

 fig. 3), shows the same condition in a less marked degree. 



