214 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Each bud, when formed, sends out two tunic vessels from its dorsal 

 side, which extend as far as the aperture of the colony. This is true 

 of the secondary zooids, but is true only of those which lie rather 

 close to the aperture of the colony. The condition in the primary 

 ascidiozooids has not been determined. 



Characteristics of the colony. The test is always of a soft and gel- 

 atinous nature, and ordinarily is transparent and colorless. The 

 test processes are very characteristic of the group, being small, sharp, 

 quadrangular spines, which always develop ventral to the zooids and 

 do not carry their oral apertures (fig. 6, pi. 18; fig. 3, pi. 16). There 

 is no diaphragm at the colonial aperture, or only a rudiment of one. 

 Some colonies bear four processes of the test projecting beyond and 

 surrounding the colonial aperture (fig. 6, pi. 18). 



The zooids. The most striking feature in connection with the 

 zooids is the oblique position of the rows of stigmata in the branchial 

 lamellae. This condition is very apparent in P. sjrinosum, (fig. 8, 

 pi. 19), the stigmatal rows, instead of being vertical, having shifted 

 over eighty degrees, until they lie almost parallel with the antero- 

 posterior axis of the zooid. In addition the branchial basket is 

 broader in the direction of the stigmatal rows than at right angles 

 to them. 



The oral aperture bears several true tentacles (fig. 2, pi. 16), in 

 addition to a median, ventral tentacle found in all Pyrosomas (fig. 

 13, pi. 22). The peripharyngeal bands come together at a sharp angle 

 considerably posterior to the ganglion (fig. 5, pi. 17). Besides the 

 luminous organs lying on each side of the pharynx, there are two other 

 cell tracts, one on each side of the cloaca, near its ventral side (Z. <?., 

 fig. 8, pi. 19), which probably have a similar function. There is no 

 blood-forming organ on the dorsal side of the body, but Neumann 

 (1909-13) believes that it is represented in this group by a mass of 

 mesoderm cells lying around the digestive tract. 1 The endostyle is 

 rather large in the members of this group, and shows histological 

 features which, though characteristic, are not mentioned here, 

 because they are of little taxonomic interest. 2 



In connection with the nervous system (pi. 16, fig. 2, and pi. 17, 

 fig. 5) there should be noted the relatively large size of the ganglion, 

 the slightly longer duct of the sub-neural gland (fig. 4, ^4), which opens 

 directly beneath the ganglion, and the distribution of the nerves. 

 This last is characteristically different in the two subgenera, as may 

 be seen by comparison of figures 5 (pi. 17) and 13 (pi. 22). The first, 

 second, fifth, and seventh nerves are the more prominent. Each 

 of the peribranchial (cloacal) muscles is innervated by a prominent 

 unbranched nerve, the seventh, while the fifth nerve goes to the 



1 Compare Salpa virgula, aggregated zooid. (Metcalf and Bell, 1918.) 



s For details of the structure of the endostyle, and figures of same, sec Neumann (1909-13). 



