REPORT ON THE TUNICATA. \<\ 



The body is always fixed, usually by the posterior end and more or less of the left 

 side ; it is in almost all cases sessile, and when not so the stalk is merely a narrow pro- 

 longation of the posterior end, and is not comparable with the peduncle of the Bolteninse. 

 The test is usually cartilaginous or gelatinous, and is found in all stages between these 

 two conditions. In the genus Chelyosoma, an aberrant form, it is developed into horny 

 plates. The number of lobes surrounding the apertures may vary considerably. In the 

 majority of cases it is — branchial eight-lobed, atrial six-lobed, bu1 the branchial is frequently 

 found with seven or with nine lobes, while in Chelyosoma it is only six-lobed ; in Abyss- 

 ascidia, on the other hand, the branchial aperture is surrounded by fourteen lobes, and 

 the atrial aperture by nine. The musculature of the mantle is generally very irregular, 

 but in the genus dona a series of conspicuous longitudinally running bands are formed. 



The branchial sac is never folded, but in many species of Ascidia it is thrown into 

 a series of minute longitudinal plications, which will be described in detail further on. 

 These must not be confused with the conspicuous folds so characteristic of the Cynthiidse 

 and the Molgulidse, which are entirely wanting in the present family. The internal 

 longitudinal bars, which are almost invariably present, are in the form of more or less 

 stout, rounded bars, in contrast to the ribbon-like vessels found in many of the Cynthiidse 

 and some of the Molgulidse. Usually in the Ascidiidse the bars bear on then- inner sides 

 papillae, varying in size, shape, number and arrangement according to the genus and 

 species. The stigmata are straight and approximately parallel, except in the genera 

 Corella, Corynascidia and Chelyosoma, where they are curved and arranged spirally 

 round certain central points. The tentacles are invariably simple, elongated, tapering 

 filaments, like those of the Styelinse, but usually rather thinner. 



The arrangement of the viscera varies considerably. In most forms, including the 

 genus Ascidia, the stomach and intestine lie upon the left side of the branchial sac ; but 

 in the nearly allied Ciona, they extend considerably beyond the branchial sac posteriorly, 

 so as to form a rudimentary abdomen. In Corella and Abyssascidia, again, the stomach, 

 intestine, and heart are placed upon the right side of the branchial sac, and the course of 

 the intestine is different from that found iu Ascidia. 



The genital glands are always found in close relation with the alimentary canal, gener- 

 ally applied to the wall of the posterior part of the stomach, or the first part of the intes- 

 tine, and often occupying the intestinal loop; they are never found attached to the mantle 

 independently of, and at a distance from, the intestine, as is so frequently the case in the 

 Cynthiidse and the Molgulidse. 



The genus Ascidia is, even in its modern restricted sense, the typical and most impor- 

 tant genus of this family. It contains by far the largest number of species, and is a 

 central point round which the other genera may be arranged, according to their affinities. 

 The first oi these is Pachychlama ; this form and Ascidia are more closely allied than 

 any other two of the genera. At the one extreme end of the series of which As. 



