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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 (b) (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 
A x 1 00 
B x 1 00 
C X 100 
D x100 
Fig. 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 . Porella e rassoparies, new species. X 100. Tangential section a little distant from the 
olocyst. Olocyst and tremocyst are visible. 
C. Hippomenella rotula, new species, X 100. Tangential section through the tremocyst. 
Areolae and tremopores are of the same nature. 
D. Porella deriticulifera, 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 onl 3 r 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 ; x the lateral arrangement 
is the cause of special manifestations quite different from those of the tremocyst. 
This deposit Harmer and Levinsen attribute entirely to the ectocyst. 
