THE PELAGIC TUNICATA. 203 



from the stolon by which it is produced. Branchial tentacles fewer in 

 number and relatively much larger in young blastozooids tlian in older 

 ones. 



Tills species differs strikingly in external appearance from any of the 

 described species of Pyrosoma, unless it be P. spinosum. The colony is more 

 cylindrical than is usual, is very slender, and especially is altogether flaccid, 

 the test lacking entirely the rigidity of that of P. ullautlcus and glganlenm. 

 The most noticeable external difference, however, is the character of the 

 common cloacal opening. 



Instead of the muscular diaphragm, which is a characteristic of all species, 

 this one is provided with four non-muscular guarding processes. The test 

 connecting these processes forms a very thin wall. The characteristic vessels 

 bearing nuiscle fibres run from the zooids down into this wall of test and 

 terminate there. No muscles could be detected in the processes themselves. 

 The processes arise from bulVj-like enlargements of the test about the common 

 cloacal opening. These enlargements are somewhat wrinkled, as though by 

 the movement of the processes, PI. I, fig. 1. It would seem that such 

 movement could only be caused by water currents, however. The spines 

 covering the test of the colony resemble those described by Herdnian for 

 P. spinomm. 



Turning to the interior of the colony, the shape of the zooids is char- 

 acteristic. In all the previously known Pyrosomae, the mature zooid is 

 elongated antero-posteriorly. In P. agassisi, however, all the zooids, young 

 and old, are much broader than long, fig. 3. These proportions obtain also 

 both for the branchial sac and the atrial chamber, as well as for the zooid as 

 a whole. The branchial sac is one third broader than long. The atrium is 

 at least four times as wide as deep. While no gonads have been found, 

 the zooids seem to be fully developed in, all other respects, but in no case 

 was there found an appreciable departure from the proportions above given. 

 The zooids are much flattened laterally. 



The musculature resembles that of P. spinosum, which the species seems 

 most nearly to approach. There are the usual muscles in the prebranchial 

 region, and besides there is a strong sphincter just above the ganglion, PI. II, 

 fig. 6. The atrial sphincter is strong, especially under the atrial tentacle, 



