382 MR. LISTER ON THE STRUCTURE AND FUNCTIONS 



In some of the last-mentioned particulars this Ascidia bears a resemblance to the 

 Sertularicc, and like them it increases by sprouts : the two streams of the stem run 

 through the bud before its organs are developed. The production of its young from 

 ovaries did not fall under my observation. No proper motion was seen in the par- 

 ticles of its blood, like that of the Sertularice. 



In a sessile Ascidia, nearly half an inch in length, of which the coat was too rough 

 and opake to allow an inspection of the branchiae, the circulation was distinctly 

 visible in the mantle near the openings, and the particles in the blood were only of 

 about the same size as in the above. 



A Polyclinum (Plate XII. fig. 1.) was met with abundantly at the same time, occurring 

 upon similar Algce, like a grey slimy crust speckled with white and black. ITie indi- 

 viduals that composed the groups were placed as if promiscuously, and without 

 arrangement. Their branchial sacs were rather smaller than those of the branching 

 Ascidia described, and considerably resembled them in general form and in having 

 four rows of spiracles with the same action of ciliae round them ; but instead of 

 being each covered by a proper envelope, they were connected by one common 

 coat that stretched over them all, and was joined to them only round the oral orifices, 

 which projected externally with a large opening and six distinct indentations : within 

 were simple tentacula, like those of the other. The vent c was placed near the base of 

 the branchial sac, with the heart h on one side, and on the other some viscera, that 

 were not well defined. Into the cavity formed by the coat the pellets of fseces were 

 discharged, and were carried away by a current that was constantly flowing in through 

 the spiracles of the branchise, and running out at a common funnel. 



Some of the funnels rose into swellings and tubes of considerable size compared 

 with that of the component animals ; and they contracted on being touched, showing 

 the coat to possess irritability. Opake whitish spots studded it here and there, and 

 encircled the openings of the funnels, and more thickly those of the mouths. The 

 thinness and transparency of the coat in the specimen drawn, were such as to show 

 distinctly within, the particles of the blood running between the spiracles of the 

 branchiae, and in one instance in the heart. They were much fewer than in the 

 Ascidia before described ; and on the coat itself I could detect no circulation what- 

 ever. 



Instead of the finger-like bodies projecting into the central cavity of the branchial 

 sac in the other, this had within, a thin ledge between each row of spiracles ; and in 

 front there were three tapering moveable prominences, one connected with each ledge, 

 that were sometimes stretched forward horizontally into the cavity, at others bent 

 downwards with a spiral curve {a, a). These seemed to suspend a generally invisible 

 vertical membrane, and to assist in giving the food its direction towards the stomach; 

 for it moved horizontally along the sides of the cavity, as in the other Ascidia, and 

 when it reached the front took a spiral motion downwards. The branchial sacs oc- 



