TUNICATA. 335 



In these figures the outer circle represents the test (f) lined by the 

 muscular mantle (t). The branchial sac in the centre {b) is perforated by a 

 few large openings which are fringed Avith ciiia ; the arrows mark the direc- 

 tion of the respiratory currents which enter at the ftral opening, passs through 

 the branchial sac into the atrium or "thoracic chamber" {c c) and escape by 

 the anal orifice {ex). 



The atrium does not exist in the embryo ; it is formed by an inflection of 

 the tunics, and its ultimate extent varies iu different genera. At first the 

 whole space between the mantle and viscera is a common vascular sinus, as 

 in the bnjozoa, but the formation of the atrium divides it into two portions, 

 one lining the mantle, the other investing the alimentary canal. The outer 

 portion, or parietal sinus, is further subdivided by the union of its walls at 

 definite points, leaving spaces and channels of various sizes and degrees of 

 regularity. Of these, the principal are the dorsal and ventral sinuses (y v') 

 communicating by transverse channels.* The lower part of the alimentary 

 canal continues surrounded by a vascular space termed the 'peri-intestlnal 

 sinus, whilst the pharyngeal portion with its vascular envelope becomes 

 perforated to form the branchial sac.f It has been mentioned that the 

 branchial openings are microscopic and innumerable in the solitary ascid- 

 ians, whilst they are comparatively few and large in the social and 

 compound species. In Salpa the branchial sac is so much reduced that the 

 respiratory process must be exercised chiefly by the vascular lining of the 

 mantle itself. 



The heart is near the posterior or fixed end of the body ; it is elongated, 

 and slightly muscular, open at each end, and contracts progressively like the 

 dorsal vessel of the anellides, the direction of its contractions being periodically 

 reversed. The nervous system consists of filaments connected with a single 

 ganglion placed in the sinus between the external orifices. :|: The organs of 

 special sense are an auditory capsule sometimes containing an otolithe, (fig. 

 225. g) and coloured spots, supposed to be rudimentary eyes, placed between 

 the segments of the outer openings. 



The neural side, or that on which the nerve-ganglion is placed, should 

 be considered ventral in these as in other invertebrate animals ; and the 

 haemal side, where the heart is situated, ought to be regarded as dorsal. § The 



* See the figure of Salpa, PI. 24, fig. 22. The thick black lines represent the 

 sinuses; the heart is near the lower end of the figure, outside the vifceral nucleus. 

 The sinuses have no visible lining membrane but resemble those already referred to 

 (pp. 31, 198) as existing in all classes of mollusea, 



t The resemblance of the pharyngeal sac of the tunicaries to the gills of fishes was 

 pointed out by Mr. Goodsir in his memoir on the Lanceiet (ampMoxus). 



t In Plate 21, the position of the nervous ganglion is indicated in several instances 

 by a small star. 



§ Milne-Edwards has employed these terms in an opposite sense, apparently 



