RETORT ON THE TUNICATA. 133 



The Test is leathery, it is tough but rather thin. The inner surface is white and 

 glistening. 



The Mantle is strongly muscular on the right side, but membranous over the 

 viscera. A narrow membrane projects into the branchial siphon above the tentacular 

 circlet ; it is sbghtly crenated, but does not bear large conical processes as in Micro- 

 cosmus helleri. 



The Branchial Sac has seven folds on each side. The transverse vessels are all of one 

 size. The internal longitudinal bars are strong and numerous ; there are about six on the 

 fold and the same number in the interspaces. The meshes are transversely elongated, 

 and contain each about twelve stigmata ; generally a fine transverse vessel divides the 

 mesh into two. 



The Dorsal Lamina is not broad, but rather thick ; the edge is plain. 



Tlie Endostyle is very broad. 



The Tentacles are about twenty in number. There are six large, six small, and some 

 intermediate very minute ones which are not present in all the interspaces. 



Tlie Dorsal Tubercle is irregularly cordate, and has both ends turned inwards. 



This species is nearly allied to Microcosmus helleri, and is not unlike it in external 

 appearance (compare figs. 1 and 5 in PI. XIV.). In the details of its anatomy, however, it 

 differs considerably, the principal points being that the present species has fourteen folds 

 in the branchial sac, while Microcosmus helleri has only twelve, and that the diaphragm 

 in the branchial siphon is different. 



The single specimen was attached by the posterior part of its ventral edge to the 

 interior of a bivalve shell (PL XIV. fig. 5). The branchial aperture is conspicuous, and 

 is directed ventrally. The test covering the siphon is not seamed and corrugated like 

 that of Microcosmus helleri; over the rest of the body it is considerably wrinkled. The 

 mantle is well developed, and forms a slight diaphragm at the base of the branchial 

 siphon, which, however, does not form large processes, as in the case of Microcosmus 

 helleri. The branchial sac (PL XIV. fig. 6) has wide meshes between the folds, contain- 

 ing each ten to fifteen stigmata. Most of the meshes are divided by a narrow horizontal 

 membrane (tr.), the wider transverse vessels (tr.) are all of one size. 



One specimen was obtained off East Moncoeur Island, Bass Strait, at Station 162, 

 April 2, 1874; depth, 38 to 40 fathoms; bottom sandy. 



Microcosmus polymorphus, Heller (PI. XIV. figs. 7 and 8). 



Microcosmus polymorphus, Heller, TJntersueh. u. d. Tun. <l adriat. Mm-., Al.th. 3. p. 0. 

 Microcosmus polymorphus, Herdnian, Prelim. Rep., Proc. Roy. Soc. Edin., 1880-81, p. 55. 

 (?) Ascidia papillosa, Cams, Beitrage zur Aiiatomie und Physiologie der Seescheiden, Meckel's 

 Aichiv., BcL ii. p. 569, pis. vii. and viii., 1816. 



