A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 241 



they still continue to open near the distal ends of the test processes. 

 As illustrating the great degree of diversity in this form, we may add 

 here that, in one of our specimens, almost all of the oral apertures 

 were situated near the bases of tho elongated spines. 



When first formed the zooids are arranged in the test with a sem- 

 blance of regularity; that is, they lie in obscurely parallel transverse 

 rows (fig. 53, pi. 36). This appearance is undoubtedly the basis of 

 Lesueur's distinction between his "P. elegans" and the other two 

 forms then recognized (P. "atlanticum" and P. " ' giganteum") . More 

 recent students believe that Lesueur was dealing with a young colony 

 of a form which we would now class as a subspecies of the species 

 P. atlanticum, probably giganteum, 1 for, in the young stages of this 

 variety, the zooids show a tendency to arrange themselves in trans- 

 verse rows. With further growth and budding within the colony, 

 (2 cm. colonies and larger) new zooids become interspersed between 

 the old ones; but not all of these arise as do the former, that is, each 

 zooid in connection with a test process. In this way, in older colonies, 

 all appearance of regularity in arrangement is lost, and the individuals 

 of the colony become crowded. 



In size the zooids show great diversity. Those in small colonies 

 (2 cm.) are of about uniform dimensions and shape (fig. 32); these 

 are from 2.5 mm. to 3 mm. in height (dorso-ventrally), from 4 mm. 

 to 4.2 mm. long when full sized. With growth, the oral-siphons 

 become enormously elongated in some of the older zooids (fig. 30), 

 and the cloacal chambers moderately so (fig. 31). The total length 

 often amounts to as much as 8 mm. The branchial sac, or basket 

 maintains a more uniform size, hence may be taken as a better 

 criterion for comparing the size of the zooids; it is found to range 

 from 3 mm. to 3.5 mm. in length when fully developed. Ritter, in 

 describing his specimens, which he called P. giganteum, gives 3 mm. 

 for the usual length of the branchial sac, and for the total length, in- 

 cluding both siphons, 5 mm. to 6 mm. The thickness of the test- 

 wall of the colony averages 4.5 mm., but may be as great as 6 mm. in 

 large specimens. The characteristic form of the zooid can be studied 

 to best advantage in the small or medium-sized specimens (6 cm. or 

 less), for, as the colonies grow larger, the individuals become highly 

 diversified. Figure 32 shows a zooid from a 1.5 cm. colony, in which 

 the contour of the body is circular to oblong, the oral and cloacal 

 chambers forming slight prominences at the ends. This same figure 

 shows also the usual relation which the zooid bears to the test- 

 process. Older zooids (fig. 30, from a 9 cm. colony) have the 

 branchial sac more elongated so as to appear oval or oblong. 



The mantle, and consequently the branchial basket, is strongly 



1 Of this identification with P. atlanticum giganteum we can not be at all certain, so can not claim the re- 

 lief such identification would give to the absurd nomenclature. 



7911—19 4 



