A TAXONOMIC STUDY ON PYROSOMA METCALF AND HOPKINS. 213 



of the second whorl, II (No. 3, fig. 9). In a similar manner a 

 fourth set of four buds is produced (No 4, fig. 9), and the two sets 

 (3 and 4) go to make up a third row (III). As many as seven or 

 eight rows may thus be established, each row containing eight zooids 

 (F, aherniosum) . At about this time, or sometimes earlier, the 

 zooids of the second row begin to bud, then those of the third and 

 fourth rows, and so on. These later buds also migrate toward the 

 open end of the colony and occupy the interspaces between the 

 zooids of the original transverse rows, establishing secondary rows. 

 In this way the regularity of arrangement of the zooids tends to be 

 lost; in some species the transverse rows persist, but in others they 

 disappear early. 



The conditions of colony-formation thus far described, are those 

 observed in the Pyrosomata ambulata. In the species P. agassizi 

 and P. spinosum, on the other hand, the buds do not migrate freely 

 through the test, so far as is known. Buds are produced in chains 

 of three or four, or even as many as seven, individuals. These tend 

 to move away from the parent zooids and from each other, probably 

 through the growth of the cellulose test separating them. From the 

 first, the arrangement of the zooids in these colonies is irregular. 

 Large colonies of P. spinosum have been observed, however, in 

 which the zooids form longitudinal rows. 



Subgenus Pyromosomata Fixata. 



The Pyrosomata fixata are sufficiently distinct from the Pyrosomata 

 ambulata to justify recognizing both as genera, but in this paper we 

 have chosen arbitrarily to treat the two groups as subgenera. 



The method of budding, so far as it is now known, is sharply distinct 

 in the two subgenera. It has been taken by Neumann (1909-13) as a 

 basis for naming the two groups. Among the fixata the buds form 

 by the constriction of a proliferating stolon, produced at the posterior 

 tip of the endostyle. As soon as one has been formed as a definite 

 bud at the tip of the stolon, others form successively between it and the 

 parent zooids. There is formed thus a chain of several small buds 

 of different sizes, as many as 7 having been observed in P. spinosum. 

 The buds do not separate until comparatively late, and there has 

 been observed no active shifting in their position, such as occurs in 

 the Pyrosomata ambulata. Each takes its place ventral to the parent 

 zooids, so that in old colonies they show a semblance of regularity, 

 lying apparently in longitudinal rows. It has not yet been deter- 

 mined just how the buds become distributed in the colony — whether 

 the primary ascidiozooids remain at the closed end of the colony, or 

 around its aperture, all the buds taking positions ventral to them. 

 Young colonies are needed for such study. 



