256 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



PYROSOMA ATLANTICUM, subspecies TRIANGULUM. 

 (P. TRIANGULUM, Neumann, 1909, a). 



Plate 29, fig. 36. 



This form was first discovered in the Indian Ocean (Deutsche 

 Tiefsee Expedition). From the careful description given by Neu- 

 mann of the single specimen captured, there can be no doubt as to 

 its relationships. During the Albatross Philippine Expedition two 

 Pyrosomo colonies were collected, which agree in about all particulars 

 with Neumann's P. triangulum; that is, the colony is conic-cylin- 

 drical, yellowish, and resembles closely the smaller specimens of 

 giganteum. The zooids have somewhat triangular pharyngeal cham- 

 bers, and are further characterized by a short, broad cloaca, a small 

 number of branchial bars and stigmatal rows, and by certain other 

 minor features. 



Among Pacific specimens of the species atlanticum we find so many 

 which approach this one in the triangular character of the branchial 

 basket that we deem it unwise to treat this as other than a subspecies. 



Our two colonies of this subspecies, one 8 cm., the other 9 cm. 

 long, differ from other Pyrosomas collected near the Philippines in 

 being more deeply colored (yellowish). They show greater affinity 

 with Atlantic forms in this respect, and also in the fact that the 

 test processes are weakly developed, short, and for the most part 

 sharply tipped. 



The zooids are irregularly arranged and closely placed. They attain 

 a length of 6 mm. (Neumann) ; in our specimen they are smaller, from 

 4 mm. to 5 mm., or averaging 4.5 mm. long; average height about 

 2.3 mm. As characterizing the subspecies, their blunt, triangular 

 form is noteworthy. This is due to the extreme shortness of the 

 branchial basket, relative to its height, and to the sharp bend which 

 the endostyle makes near its anterior end. This effect is increased 

 because the cloaca, and the viscera lying anterior to it (the gut and 

 testis), are displaced somewhat toward the dorsal side of the animal. 

 In each branchial lamella the number of longitudinal bars is pretty 

 constantly 13-14, of stigmatal rows 25-27. There are from 6 to 8 

 dorsal languets. The cloaca is rather broad, the cloaca! muscle 

 long. The testis, consisting of about 15 lobes, shows no unusual 

 conditions. 



One of our specimens presents a characteristic condition of pro- 

 gression in the manner in which the sex cells mature in different parts 

 of the colony. At the middle, and near the closed end of the colony, 

 the majority of the zooids are protandrous; around the open end of 

 the colony practically all are protogynous. So we have here another 

 of those forms which are referred to as protandrous — meaning, of 

 course, that there is a preponderance of protandrous zooids within 

 the growing colony. 



