REPORT ON THE TUNICATA. 131 



T am now of opinion, however, that as these characters, and especially the condition of 

 the dorsal lamina, are apparently constant and are easily applied, and as the two groups 

 of species (Microcosmus and Cynthia) seem fairly distinct, it is, if not absolutely necessary, 

 at least convenient to retain both generic terms. 



The Challenger expedition added two new species to the nine or ten already known, 

 and found a large specimen of Microcosmus polymorphus, the common Mediterranean 

 species, in a new locality. 



Microcosmus helleri, Herdman (PI. XIV. figs. 1-4). 



Microcosmus helleri, Herdman, Prelim. Rep., Proe. Roy. Soc. Edin., 1880-81, p. 54. 



External Appearance. — The body is longish ellipsoidal in shape, with a projection at 

 the anterior end ; it is scarcely compressed laterally. It is attached by a small area at the 

 posterior end of the ventral edge. The anterior end narrows rapidly into the truncated 

 conical branchial projection ; the posterior end is broad and rounded, and the dorsal and 

 ventral edges are nearly straight and parallel. The branchial aperture is at the anterior 

 end of the dorsal edge; it is terminal, on a large projection directed anteriorly and slightly 

 ventrally. The atrial aperture is on the dorsal edge, about two-thirds or three-fifths of the 

 way down, and placed on a hemispherical projection. It is not so prominent as the 

 branchial aperture, and is directed dorsally and a little posteriorly. 



The surface is wrinkled and roughened, but not covered with excrescences ; the 

 branchial and atrial projections are much corrugated and thickened ; a few Zoophytes, 

 Polyzoa. &c, are found adhering, especially on the left side and posterior end. 



The colour is a dull gamboge yellow, with a little reddish-brown at the posterior end 

 and on the left side. 



Length of the body, 8 cm. ; breadth of the body, 4 cm. 



The Test is leathery; it is rather thin, except at the area of attachment and on the 

 siphons. The inner surface is white with a few yellowish-brown patches. 



The Mantle is strongly muscular on the right side and the dorsal part of the left; it is 

 membranous on the ventral part. The musculature is strong and regular. 



The muscular band at the base of the branchial siphon, and just above the circle of 

 tentacles, bears four large bluntly conical processes projecting into the lumen of the tube. 



The Branchial Sac has six folds on each side. The alternate transverse vessels arc 

 larger than the intermediate ones, and about every fifteenth larger transverse vessel is very 

 much wider than the others. The internal longitudinal bars are numerous. There are 

 eight only on the folds and about twelve in the interspaces, each of which has six wide 

 and six narrow rows of meshes. The largest meshes contain each six to eisdit stigmata. 



Tie Dorsal Lamina is a plain membrane. 



The Tentacles are compound, they are twenty in number, and of two sizes placed 

 large and small alternately. 



(ZOOL. CH^LL. EXP. — PART XVH. 1882.) R 18 



