378 MR. LISTER ON THE STRUCTURE AND FUNCTIONS 



and at others softened or dissolved ; but it is not altered by absorption of the soft 



parts. 



ASCIDIiE. 



The small Ascidia, Plate XI., fig. 1 to 7, was not unfrequent at Brighton, in August 

 1833, on pieces of Conferva elongata that were washed ashore, and had to the naked 

 eye the look of minute lumps of pellucid jelly with a spot of orange and grey. It 

 does not, I believe, come within either of the descriptions of subgenera of Savigny 

 or MacLeay. 



It occurs in groups that consist of several individuals ; each having its own heart, 

 respiration, and system of nutrition, but fixed on a peduncle that branches from a 

 common creeping stem, and all being connected by a circulation that extends 

 throughout. Their parts are of such transparency that their interior is easily seen. 

 Their external shape is that of a pouch compressed at the sides, and fixed at the 

 hind part of its base upon the peduncle. 



Its two openings are in the form of very short tubes ; that of the mouth g at the 

 top of the pouch, and that of the funnel /' in front *. The longest diameter, from 

 the peduncle to the space between the openings, is about -085 inch. 



The outer covering is a tough coat, a, a continuation of the peduncle, more pliable 

 near the openings ; lined internally with a soft substance or mantle h, in which a rami- 

 fying circulation is very distinct. A great part of the interior is occupied by the 

 branchial sac c, which is subcylindrical, flattened at the sides, and has its axis vertical; 

 its cavity terminating upwards in the oral opening, and being closed at the bottom. 

 It is united to the envelope or to the mantle above and behind ; the juncture, c' c' be- 

 ginning in front of the oral opening, extends backwards on each side of it, and then 

 downwards in two lines : between these, along the middle of the back, is a vertical 

 compound stripe d (fig. 4), that seemed to me cartilaginous. At the bottom the sac 

 appears to be enveloped by the soft substance of the mantle, but at its sides and front 

 a vacant space is left between them, that ends in the opening of the funnel. The 

 branchial sac is more compressed towards its lower part ; and here are placed, ex- 

 ternally to it, the heart m on the left, and the stomach i and other viscera on the right 

 side, the vent h opening upwards at the front into the funnel. On its sides and front 

 the sac is perforated by four rows of narrow, vertical, irregularly oval holes or 

 spiracles, about sixteen in each row, placed at less than the diameter of one apart 

 from each other. Through these the water, which flows constantly in at the mouth 

 when its orifice is open, appears to be conveyed to the vacant spaceybetween the sac 

 and mantle, and it then escapes at the funnel. The sac seems extremely thin be- 

 tween the spiracles ; but their edges are thickened, as if cartilaginous ; and they are 

 lined with closely set cilice, which, by their motion, cause the current of water. When 

 these are in full activity (fig. 7), the eflfect upon the eye is that of delicately-toothed 



* The terms back and front as they stood in my memoranda are here interchanged, to accord with the 

 designation of the parts given by Savigny and Cuvier. 



