A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 201 



ticum and subspecies, pi. 34). P. spinosum acquires a degree of reg- 

 ularity in the arrangement of its zooids, but only after it becomes 

 large and mature. It then contains longitudinal rows of zooids, those 

 of one row alternating with those of the next. In P. verticillatum 

 and a few related species the regularity arises with the earliest bud- 

 ding of the zooids, these coming to lie in both longitudinal and trans- 

 verse rows (figs. 43 and 44, pi. 33). The longitudinal rows soon 

 become obscured, but the transverse rows (verticils) persist. 



The older distinction for dividing the genus into two groups 

 (Savigny, 1816) according to the arrangement of the zooids in the 

 colony, whether regular or irregular, appears not to have been well 

 grounded. The form designated as "P. elegans," whose zooids were 

 regularly arranged, was probably a young specimen of some form of 

 the species we treat as P. atlanticum, since some of these forms are 

 known to pass through a stage in which the zooids are arranged 

 with a considerable degree of regularity. This original distinction 

 should not be confused, however, with that which now holds good 

 in separating the so-called verticillate forms from the others of the 

 genus. These have all been discovered very recently (1909 and 

 subsequently) . 



The test. The outer surface is never quite smooth and even. The 

 embryo, at least in several species, when first released from the parent 

 colony, developes spine-like outgrowths of the test, which apparently 

 aid in suspending the young animal in the water. These are purely 

 embryonic structures, however, which always disappear after the 

 new colony has commenced active growth through budding. In 

 many species of Pyrosoma true test processes are present on the adult 

 colony. These processes are of two distinct types. Those character- 

 istic of the group Pyrosomata Jixata are small, quadrangular spines, 

 located on the test surface, just ventral to the oral apertures of the 

 zooids (fig. 6, pi. 18 and fig. 8, pi. 19). Originally they point slightly 

 in the direction of the open end of the colony, thus giving one the im- 

 pression that they slightly overarch the oral apertures of the zooids. 

 The test processes of the other type, which are found in some species 

 of Pyrosomata amhulata, always occur in connection with zooids; 

 that is, each test process surmounts a zooid, whose oral siphon 

 traverses the length of the test process to open by the mouth upon its 

 truncated ventral surface, or at its distal end (fig. 45, pi. 34, and 

 fig. 30, pi. 26). This relation often gives rise to long buccal siphons 

 surrounded by the tubular outgrowths of the test (as in P. ovatum, P. 

 ahemiosum, and others. See fig. 26, pi. 25). There are no other 

 definitive spines or processes found in any members of this sub-genus, 

 which originate independently of the zooids. Irregularities and 

 denticulations of the test are found, but they are, for the most part, 

 unimportant. In some cases this roughened or denticulate condition 



