50 



BULLETIN 106, UNITED STATES NATIONAL, MUSEUM. 



and Levinsen have carefully studied these structures. " These tubes have a disk 

 near the junction with the zooecium. and in the middle of the axial tubes, which 

 are often very long, there are similar disks (fig. 7, A). The shell structure is 

 terminated by a membrane (&) (fig. 7, B) perforated by these pores; but over this, 

 as in the greater part of the cheilostomatous bryozoa, there is another fairly 

 thick and somewhat chitinous membrane (a) (fig. 7, B) continuously covering 

 the zoarium. The cell contents of the tubes attach themselves to this by delicate 



AxlOO 6x100 CxIOO 0x100 



PIG. 6. Microstructure of the tremocyst and olocyst. 



A. Stomachetosella crassicollis Canu and Bassler, 1917. Tangential section, X 100, at the 

 level of the subjacent olocyst ; the inner orifice of the tremopores is visible. 



B. Piii-cllii n-iixKiiiiancs, new species. X 100. Tangential section a little distant from the 

 olocyst. Olocyst and tremocyst are visible. 



0. Hiiiiiuiiii -IK llu rutiila. new species. X 100. Tangential section through the tremocyst. 

 Areolae and tremopores are of the same nature. 



D. Porella denticulifera, new species, X 100. Tangential section through the orifices of the 

 tremopores. 



threads" (fig. 7, C) (Waters, 1900). Levinsen in 1909 demonstrated that these 

 " delicate threads " were formed of mesenchymatous tissue. 



Pleurocyst. The areolae are only lateral tremopores; they are the remains of 

 endocystal buds and are traversed by mesenchymatous fibers. The superior endocyst 

 continues its calcareous deposit as the pleurocyst; l the lateral arrangement 

 is the cause of special manifestations quite different from those of the tremocyst. 



1 This deposit Harmer and Levinsen attribute entirely to the ectocyst. 



