202 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



gives to the colony a peculiar opacity. It is occasionally of some 

 taxonomic value, as will be noted later. 



Certain other structures should be described here, on account of 

 their relation to the colony as a unit, namely: the vessels of the tunic 

 and the fibers connecting the cloacal muscles of adjacent zooids. 

 Discussing the latter first, it is probably by means of these fibers that 

 contractile movements among the individuals composing the colony 

 are coordinated, and it may be that the simultaneous action of the 

 cloacal muscles (and the fibers ?) is effective to cause contraction of 

 the colonial tube. In any case their action, whether contracting only 

 the individual cloacal chambers, or contracting the whole colonial 

 tube, must bring about locomotion by the expulsion of water from 

 the colony through the aperture at its open end. The longitudinal 

 libers of the tunic vessels may share in this effect . In the performance 

 of this function there must be some nervous coordination, some means 

 by which the cloacal muscles in all the zooids are stimulated to con- 

 tract simultaneously. Since these test fibers are the only connections 

 between the zooids, they must be the carriers of nervous impulses 

 from one individual to another. 



The tunic vessels, above mentioned, appear in the earliest buds of 

 the colony, as ectodermal outgrowths from their dorsal walls. Two 

 are found in connection with each of the four primary ascidio zooids 

 probably of all the species, and with the subsequently formed zooids 

 of the Pyrosomata Jixata. In the other subgenus, however, the sec- 

 ondarily developed buds send out but one vessel each. The tunic 

 vessels are cylindrical tubes consisting of a single epithelial layer lined 

 with longitudinal musle fibers. They all end blindly in the diaphragm 

 of the colony when this is present . They serve the purpose of a colonial 

 circulatory system. In young colonies these vessels are well devel- 

 oped and functionally active, but in adult colonies they often undergo 

 degeneration, especially the enormously elongated vessels which arose 

 in connection with the oldest zooids. 



The test is rich in test-cells. These originate in the mesoderm of 

 the early formed buds and wander out through the ectoderm to their 

 place in the test. 



The zooids (fig.l,pl. 15) are numerous and almost independent, being 

 held together in a common matrix, the test. They may be examined 

 most readily by cutting out thin sections of the wall of the colony, 

 parallel to its long axis. The outer ectodermal sheath, the epidermis, 

 is attached to the test only at the oral and atrial apertures. As in the 

 other Tunicates, there are two principal body chambers, a pharynx, 

 and an atrium with its two large peribranchial pouches. Between 

 these two chambers, postero-ventrally and near the mid fine, lie the 

 viscera, namely, the digestive and reproductive organs and the heart. 

 The muscles of the zooid (fig. 8, pi. 19) are only weakly developed. 



