A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 207 



for the branchial vessels open both above and below into longitudinal 

 vessels which lie parallel to the ventral and dorsal sinuses. There are 

 two on each side, one above and one below, following the dorsal and 

 ventral contours of the branchial basket. The two ventral, delimit- 

 ing, branchial vessels (v. I. s.) are connected by numerous ducts with 

 the median ventral sinus, and the two dorsal (d. I. s.) are similarly 

 connected with the median dorsal sinus. In the region of the stomach 

 the dorsal sinus breaks up into a ramifying system of branches, 

 which originated in the bud from a large, undivided canal, the visceral 

 sinus (vc. s). One branch traverses the stomach and intestine 

 before reaching the heart; others unite posteriorly to form a genital 

 circuit, (g. s.), which goes to the gonads, and thence to the heart. 

 The growing stolon is supplied with two vessels, one from each end of 

 the heart. Two small branches of the ventral sinus, one to the oral 

 region (v. a. s.) and one to the cloaca, receive blood from two corre- 

 sponding branches from the dorsal sinus (d. a. s. and d. p. s.). The 

 tunic vessel (t. s.) is supplied with blood from the dorsal sinus, but 

 the blood does not circulate in it. It is through these tunic vessels 

 that some of the niesenclryme elements pass into the cellulose test. 



Blood-forming organs. On the dorsal side of the zooid, some dis- 

 tance in front of the esophageal aperture, there are two elongated 

 masses of mesoderm cells which seem to be leucocytes, lying in an 

 expanded portion of a blood sinus (pi. 15, b. o.). The function of 

 these cell masses is uncertain. They were formerly thought to be 

 glandular. Seeliger (1895) found them to contain many actively 

 dividing blood corpuscles, and believes them to be organs for the 

 formation of blood cells. 



The luminous organs lie one on each side of the pharynx, just 

 over the peripharyngeal bands (pi. 15, I. o.). They are flat, oval 

 masses of mesodermal cells, contained within an enlargement of a 

 blood sinus. They resemble the blood-forming organs in general 

 appearance and structure and in their relation to the blood stream. 

 Their method of producing light is not well understood, though it 

 is probably by the oxidation of a yellowish granular substance within 

 the cells, as seems to be the case in the luminous organs of Salpa. 

 Light is also emitted from cells in the region of the intestine and 

 gonads. 



Gonads. Pyrosoma is hermaphrodite. The organs of reproduc- 

 tion are found in the ventral body wall, below the digestive tract. 

 In the Pyrosomata fixata the testis is closely applied to the postero- 

 ventral side of the digestive tract (pi. 19, fig. 8, t.), and the egg (ov.) 

 lies considerably posterior to it. In the other Pyrosomas the testis 

 is at some distance from the digestive tract, the egg developing at 

 the right of the testis (fig. 31, pi. 27). In species with small colonies 



