234 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



of the zooid while water is at the same time entering it from the 

 mouth. 



PYROSOMA AHERNIOSUM (Seeliger. 1895). 



Plate 24, figs. 24 and 25. 



The German Plankton Expedition found this species of Pyrosoma 

 in the tropical regions of the Atlantic, where it appears to be not at 

 all uncommon. Forty-eight colonies were collected, from nine dif- 

 ferent stations, by this expedition, while others have been found 

 subsequently in the Indian Ocean. The species is not represented in 

 the collections made by the United States Bureau of Fisheries, hence 

 we are dependent for our description upon Seeliger's original account 

 and Neumann's later studies of the same form. 



The form of the colony is stated as variable, being usually cylin- 

 drical or conical, the open end being broader than the closed end. 

 In young colonies with but few whorls of zooids the closed end may 

 be considerably the broader. The maximum length is 3 cm., the 

 width 14 mm., although it is possible, as Seeliger thinks, that larger 

 specimens will be found. Since no specimens over 3 cm. long have 

 been reported (Deutsche Siidpolar, and Deutsche Tief see-Expedi- 

 tion), it seems very probable that this represents about the definitive 

 colony length for the species, especially since the zooids in colonies 

 of this size are sexually mature. There is a regular arrangement of 

 the zooids in the young colonies with 1 to 3 whorls of zooids, but in 

 older colonies this is completely lost. The definitive test processes 

 in this species resemble somewhat those of P. ovatum, except that they 

 are much shorter (fig. 24). In form they are thick-walled tubes, 

 which surround the oral siphons of the zooids and stand out perpen- 

 dicular to the surface of the colony. At their ends the oral apertures 

 are sunken in crater-like depressions and usually face directly out- 

 ward, sometimes a little ventrally. 



The maximum length of the zooid is 5 mm. The oral siphon is 

 fairly long, almost equaling the rest of the body in some cases. Its 

 great width is significant also, the anterior wall being broad and flat. 

 On the inner (endodermal) wall of the oral siphon pigment cells have 

 been found, extending as far forward as the mouth (Neumann, 1913, 

 b). The branchial basket is broad (dorso-ventrally) , about equally 

 so at both its ends. The endostyle is only slightly curved. There 

 are about 24 rows of broad stigmata, and ordinarily 14 longitudinal 

 bars. The cloaca is a little narrower than the pharynx, the cloacal 

 muscle rather long. Pigment cells are found on the outer epithelium 

 of the cloaca (fig. 25), as well as on the follicle wall of the testis. 

 The testis consists of about 12 largo lobes which surround the intesti- 

 nal loop. The organ does not lie in an evagination of the ventral 

 body cavity, as is ordinarily the case in other Pyrosomas. The ovary 

 occupies its usual position at the right side of the testis. 



