200 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



drical, usually tapering somewhat toward the closed end. The 

 zooids are found to be arranged radially around the central axis. 

 At the open end of the colony the aperture is bounded by a diaphragm, 

 a shelf like continuation of the test, serving to narrow the aperture. 

 This diaphragm contains numerous spindle cells, which for the most 

 part are arranged in circles parallel with the free edge of the dia- 

 phragm, and have even been observed by Seeliger to form definite 

 circular bands. Whether these are contractile or not is unknown. 

 Seeliger thinks them merely elastic, but the similarity between these 

 cells and those at the frayed-out ends of some of the muscles of the 

 zooids suggests strongly that they may be muscular, and effective in 

 contracting the colonial aperture. 



The thickness of the test walls is nearly uniform for each colony, 

 and this corresponds roughly to the normal length of the zooids; it 

 ranges from about 0.2 cm. to 2.0 cm. This maximum thickness is 

 reached only in large colonies of P. spinosum. Often the test is color- 

 less and transparent, but in many preserved specimens it is slightly 

 colored — yellowish, blue-green, grayish or flesh colored, the colony as 

 a whole appearing darker because of the opacity of the zooids. 



The test may be soft and gelatinous or rigid and cartilaginous, in 

 some forms even leathery. Usually it is fairly firm and gives to the 

 colony some definite form, which may be characteristic of the species. 

 It appears that in the subgenus Pyrosomata fixata the test is always 

 flabby and gelatinous; even alcoholic specimens, which ordinarily 

 are quite hard, in this subgenus are surprisingly limp. 



The arrangement of the individual zooids can be seen from a 

 superficial examination of the entire colony. The zooids, with only 

 occasional exceptions, are disposed with their dorsal sides directed 

 toward the open end of the colony. In some of the larger forms (P. 

 atlanticum gignateum, P. ovatum) it is not uncommon to find some 

 zooids turned around, so that their ventral sides are directed toward 

 the colonial aperture. Keferstein and Elders (1861) described this 

 position of the zooids as a diagnostic character of the species "P. 

 elegans," but this seems unwarranted, since it is found, to a greater 

 or less degree, in a number of species, and seems to be due merely to 

 overcrowding of the zooids. Seeliger thinks Keferstein and Ehlers 

 may have been working on broken specimens, mistaking a broken 

 end for the open end of a colony; or that perhaps they examined P. 

 aherniosum colonies, which are sometimes larger at the closed than 

 at the open end, and that they overlooked the position of the aperture. 

 An orderly arrangement of the zooids obtains in some species. In 

 the young colonies of almost all types there is some degree of regu- 

 larity (fig. 53, pi. 36). This may persist throughout life in the 

 smaller, verticillate forms, but generally in a modified way (fig. 54, 

 pi. 36); or it may become completely obliterated in others (P. atlan- 



