334 MB. A. HAKCOCK ON THE AKATOAIT 



rupted by intervaseular communications. Eadiating vessels, how- 

 ever, which are sufficiently numerous to prevent engorgement, 

 pass from the apex of each cone to the transverse vessels, and 

 are the principal interruptions to the continuity of the spiral 

 stigmata. The oral lamina in this, as in all the members of the 

 genus, is a plain simple membranous band. 



AH the simple Ascidians that have come under my notice, not 

 even excepting Pelonaia, have a collar of tentacular filaments situ- 

 ated at the base of the incurrent tube, some distance above the 

 entrance of the branchial sac ; indeed the distance in some species 

 is considerable, and no instance has occurred in which they could 

 be said to be connected with the giU. They are usually linear or 

 slightly conical, and are rather numerous, except in Pelonaia, which 

 has not more than twelve or fourteen ; but in Molgula and in 

 some of the other Cynthiadcd they are branched or pinnate, and 

 are not very abundant. They, however, all agree in being soft, 

 delicate, hollow organs ; and the simple ones ,at least, have the 

 interior divided by a septum into two longitudinal channels, so 

 that the blood will circulate freely through them. They appear 

 to be an outgrowth of the lining membrane, and are supplied with 

 blood from that which flows between it and the mantle or inner 

 tunic. 



That enigmatical organ the branchial tubercle (the anterior 

 tubercle of Savigny) is situated in the space between the tenta- 

 cular filaments and the anterior margin of the branchial sac, in 

 contact with the upper membrane of the anterior cord or collar, 

 and immediately in front of the upper extremity of the oral or 

 ventral lamina. It is formed of two parallel folds of the lining 

 membrane pressed close together and united at the extremities ;- 

 they seem but as one fold, and are bent into a loop with the ends 

 turned towards the inhalant orifice, and, inclining inwards, are a 

 little convoluted. Thus the organ assumes a rounded or oval 

 form, rising above the surface to which it is attached as a depressed 

 compact tubercular swelling. An opaque white line marks the 

 separation of the two folds, and follows the convolutions to the 

 extremities. 



This is the form that this curious tubercle assumes in Ascidia 

 scahra, A. affinis, a closely allied species, A. mentula, and Pelonaia 

 corrugata. In A. sordida one of the extremities turns inward, 

 the other outward, so that both are bent in the same direction. 

 But more striking modifications occur in some other species. In 

 one allied to A mentula there are three loops, crowded upon each 



