A TAXONOMIC STUDY ON PYROSOMA METCALF AND HOPKINS. 221 



expect, a point not definitely ascertained by other workers, on account 

 of the poor condition of their material. 



The mantle is thin and delicate. 



The musculature is fundamentally similar to that in P. agassizi, 

 but is much more highly developed (pi. 19). The fibers of the lateral 

 muscular system are numerous and complexly interwoven. The dor- 

 sal portion, consisting of 8-10 long branches, is connected distally on 

 each side of the animal, by two or three strands, with the ventral 

 portion, which has about 4 branches. Some of the dorsal fibers are 

 fused with the thick cloacal muscle. Around the oral aperture, out- 

 side of the sphincter, are an indefinite number of circum-oral fibers 

 which are simple on the ventral side, but dorsally branch and anasto- 

 mose freely with each other and with the oral sphincter. Their 

 maximum number is 7 or 8. The cloacal muscle, over the peri- 

 branchial region, is short and thick. It runs nearly parallel with 

 the long axis of the colony. An atrial sphincter is present on the 

 dorsal edge of the cloacal aperture as a broad band. It does not 

 extend more than halfway around on each side of the aperture. 



In this species the prebranchial chamber is more prominent than 

 in P. agassizi, but is never prolonged into a siphon. The oral aper- 

 ture is a little sunken beneath the surface of the test. It bears a 

 circle of long tentacles, about 15 in number, of variable size, the 

 median ventral one longer than the rest. 



The branchial chamber proper is a greatly elongated cavity, whose 

 stigmatal rows lie at an angle of about 10° to the longitudinal axis 

 of the zooid. It is elliptical in contour and a trifle narrower at the 

 anterior than at the posterior end. Each of the delicate branchial 

 lamellae bears about 55 narrow stigmatal rows and a large number of 

 bars, 30-44, which in this species run nearly transverse to the prin- 

 cipal axis of the zooids. The anterior third of the endostyle is curved 

 sharply around the front of the pharynx, so that this portion of it 

 really bounds the anterior end of the zooid. From its forward end 

 the peripharyngeal bands pass diagonally toward the ganglion, con- 

 tinuing distinct posterior to it for quite a distance. At the point 

 where they unite the dorsal languets commence. There may be as 

 many as 22 of these (Neumann). We find in our Pacific specimens 

 only 9 or 10, all rather long and slender and inconspicuous. 



The luminous organ is a small indistinct group of cells, of oval 

 form, lying over the peripharyngeal band and very near to the 

 endostyle. Luminous organs also are found on each side of the 

 cloaca, near the ventral end of its aperture (Z. o'. in figs. 8 and 9). 

 They are in the form of thin groups of loosely arranged colls. 



The ganglion is egg-shaped. The duct to the ciliated funnel turns 

 sharply backward before making connection with the wall of the 

 pharynx. 



