216 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



directly continuous, the arrangement over the middle of the body 

 being confused by the interruption of most of the ridges and the sup- 

 pression of others. In some of the larger specimens the processes at 

 the open end of the colony are entirely lacking. It may be that we 

 have here a retrograde variety characterized by the absence of these 

 processes. In addition to the ridges, there are always present on the 

 test the small quadrangular spines so characteristic of members of 

 this subgenus (fig. 6, pi. 18). If anything, they are a little less numer- 

 ous than in P. spinosum. On some colonies they are rather incon- 

 spicuous; the largest reach 5 mm. in length (Ritter and Byxbee). 

 The relative dimensions of the colony do not vary widely. Small 

 colonies are more fusiform, or taper more toward the closed end, than 

 do large ones. All our preserved specimens are flattened except for 

 one or two very young colonies. We can not be at all certain whether 

 this flattening is natural or due to collapsing in preservation because 

 of the extreme softness of the test. The colony is described as cylin- 

 drical by Ritter and Byxbee, who state that the diameter of a 12 cm. 

 colony is about 1 cm. Our larger specimens do not exceed 8-11 cen- 

 timeters in length, and this seems to be the maximum length attained 

 by the majority of specimens of this species, according to other 

 writers (see Neumann, 1909-13, p. 13), but since no colonies of this 

 species bearing sexually mature zooids have been reported, larger 

 specimens may be found. Possibly at the time of sexual reproduc- 

 tion this Pyrosoma lives customarily at considerable depth. 



The zooids are arranged in the test irregularly (fig. 42, pi. 33). It 

 is possible that they assume a more regular arrangement in longi- 

 tudinal rows in the oldest colonies, as this is the case in the nearly 

 related species, P. spinosum. The dimensions of our largest zooids, 

 in each of which is visible a small mass of cells between stomach and 

 intestine, probably the forerunner of the testis, are as follows: 

 length, as measured from the mouth to the atrial aperture, 2.5 mm- 

 3.2 mm. ; height, measured dorso-ventrally at right angles to the longi- 

 tudinal axis, 2.5 mm.-3.5 mm. From these figures it will be seen 

 that the zooid is higher than long. This appearance is emphasized 

 by the fact that the disproportionate height of the branchial basket 

 is even more marked, it being elliptical with the greater axis vertical. 

 The oral and cloacal chambers are very much reduced. The rectum 

 opens almost directly into the common colonial chamber because of 

 the shallowness of the cloaca. 



The mantle is a thin, delicate lamella in which the cell layers, 

 ectoderm, endoderm, and mesoderm can scarcely be distinguished, so 

 intimately are these fused together. The muscles associated with 

 the mantle may be roughly assigned to three systems more or less 

 independent of each other. Those of the oral region consist of an 

 oral sphincter, and outside of this a variable number, three or four, 



