338 
ARTHUR DENDY. 
(1) The Walls of the Inhalant and Exhalant 
Canals. 
It ■will be convenient to give these the first consideration on 
account of their close relationship to the ectosome, from which 
they cannot be sharply separated. 
The larger or proximal portions of the inhalant canal system 
are provided with special walls of mesodermal tissue. The 
true nature of this lining membrane is difficult to determine. 
In sections it is seen to consist of a very much vacuolated 
gelatinous tissue, composed more or less of cystenchyme, but 
in parts becoming fibrous. 
No doubt even the larger portions of the inhalant canal 
system are also provided in life with a very delicate epithelial 
lining, forming the outermost layer of their walls, but this I 
have not been able to detect, possibly owing to the manner in 
which the entire gelatinous lining membrane shrivels up in 
spirit. 
The ultimate inhalant lacunae, which open into the flagellated 
chambers as above described, have no special mesodermal walls ; 
but here the nuclei of a delicate, flattened, lining epithelium 
can be easily detected in thin sections (fig. 6, n. e. c.), and 
occasionally a large cystenchyme cell may be seen embedded 
in the choanosome immediately beneath this epithelium. 
The ultimate exhalant lacunae, into which the flagellated 
chambers discharge their contents, have, like the ultimate 
inhalant lacunae, no special mesodermal walls, but are lined by 
a delicate flattened epithelium, whose nuclei can be detected 
in thin sections. The larger exhalant channels, or oscular 
tubes, are, however, provided with most distinct walls, which 
can be dissected away from the underlying tissues with great 
ease (fig. 5). These walls are membranous and fairly tough, 
and they are continued from the oscular tube itself along its 
various larger branches as a distinct lining membrane. 
The wall of the oscular tube is seen in transverse section to 
be made up of the following layers from within outwards : 
(a) A thick rather irregular layer of very much vacuolated 
