So WILLIAM BLAXLAND BENHAM. 
wise exist by virtue of the dorsal gap in the inner series of 
tentacles. 
The epistome is regarded by Caldwell as a small remnant of 
the pre-oral lobe of the larva. 
Its histological structure is shown in fig. 11. Its dorsal sur- 
face is covered by a cubical epithelium continuous with and 
similar to the surrounding epidermis. Its oral surface agrees 
with the epithelium of the cesophagus, and consists of very 
elongated, narrow columnar cells carrying cilia (o7.). At the 
base of these is seen nervous tissue. The epistome is hollow, 
the ccelom being continued into it, but this space is traversed 
by a skeletal tissue which passes from the basement tissue 
below the epithelium of the two surfaces. This skeletal 
tissue resembles the septum (see below), and is of the nature 
of cartilage. 
The Body Wall.—The body wall is formed of (1) epi- 
dermis, (2) basement membrane (or better ‘‘ basement tissue” 
of McIntosh), (3) circular muscles, (4) longitudinal muscles, 
and (5) coelomic epithelium (fig. 9). 
The epidermis consists chiefly of columnar, cuticle-secret- 
ing cells with pigment granules, and interspersed are a fair 
number of gland-cells, with spheroidal contents, like those in 
the “ granule-cells ” of the Earthworm’s epidermis. In the 
oral region the pigment granules are so numerous as to 
render the outer part of the epidermis nearly black. 
The basement tissue is an almost homogeneous layer 
of considerable thickness, immediately below the epider- 
mis; here and there can be found a cell, usually spindle- 
shaped, embedded in this tissue. It is continuous with the 
septum, and varies in thickness in different parts of the body, 
so that we are justified in considering it as being elastic. This 
layer is thinnest over the area of the rectal and nephridial 
ridges; and it is due to the thinness, probably, that these 
ridges are apparent; the tubes being able to push out the 
body wall. ° 
The muscular layers are not very thick; the fibres are 
unstriated. The longitudinal muscles are arranged, not con- 
