A TAXONOMIC STUDY ON PYROSOMA METCALF AND HOPKINS. 205 



the zooid; in a few species they are higher (dorso-ventrally) than 

 long (as P. agasslzi, fig. 3, pi. 16; P. verticillatum, fig. 12, pi. 21). 



In Pyrosoma the organs of digestion resemble very closely those 

 in other Tunicates. They are located centrally in that portion of 

 the body wall which forms a partition between the pharynx and the 

 cloaca. The long axis of this rather open "nucleus" is perpendicu- 

 lar to the antero-posterior axis of the zooid in most cases; in P. 

 spinosum, however, it is distinctly oblique (fig. 8, pi. 19). The esopha- 

 gus is a short, funnel-shaped tube in all species. It opens from the 

 pharynx at its posterior end near the dorsal side, then curves down- 

 ward, entering the stomach posteriorly (fig. 25, pi. 24; fig. 4, pi. 17). 

 This latter organ is cylindrical, ovoid or. triangular. It is sharply 

 demarcated from the esophagus, but is not so distinct from the intes- 

 tine. Between the stomach and the true intestine is a short, con- 

 stricted portion, which may be called a pylorus (figs. 17 and 18, pi. 23) . 

 The intestinal tube runs ventrally, then curves forward and upward, 

 and opens at the left side of the stomach into the cloacal chamber. 

 The intestinal curve is a sharp loop in some species (fig. 17, pi. 23), 

 while in others it is more evenly curved, or almost circular (fig. 34, 

 pi. 28). In the Pyrosomatafixata the intestine curves sharply upward 

 immediately after leaving the stomach (fig. 7, pi. 18; fig. 8, pi. 19). 

 There is a digestive, or pyloric, gland as in other Tunicates. It lies 

 over the distal limb of the intestinal loop as a branching system of 

 tubules. These converge to form the common duct which enters the 

 stomach on the left side near its ventral end (fig. 27, pi. 25). 



The peribranchial chambers, one on each side of the pharynx, cor- 

 respond pretty closely in contour with the branchial lamellae (fig. 3, 

 p. 204.) They are flattened diverticula of the common atrium, extend- 

 ing forward just far enough to receive the exhalent water coming 

 through the stigmata from the pharynx. The lateral walls between 

 pharynx and atrium are very thin, consisting of endoderm, ectoderm, 

 and but little mesoderm, abundant blood lacunae, however, being 

 present. There are delicate trabeculae connecting the inner wall of 

 the peribranchial chamber with the outer one. 



The common cloaca, that is, that portion of the atrial chamber lying 

 posterior to the pharynx, is of variable size, forming in most species a 

 considerable cavity (pis. 30 and 31), but in some species it is but a 

 shallow bay opening by a very wide cloacal aperture into the central, 

 colonial chamber (fig. 3, pi. 16). Into the cloaca the sexual elements, 

 the faeces, and the respiratory water are discharged; and it undoubt- 

 edly serves, in part at least, to conduct away the products of excre- 

 tion, though there are no well defined renal organs in Pyrosoma. In 

 this cavity, also, the embryo, in some species, continues its develop- 

 ment for a period after becoming detached from the ovary. On 

 account of its great diversity in size and structure, no condition of the 



