206 
W. A. HASWELL. 
toneum. From this we might infer that a distinct separate 
layer of such cells lines the coelom, but the figures of sections 
given in the plates — e. g. the figures on PI. 7 — do not show 
this, and represent only a single set of nuclei. 
But there are also adult forms among the Chaetopoda in 
which the condition is remarkably like that occurring in 
Stratiodrilus. In an Enchytraeid not yet determined, 
which is very common in Sydney in moist garden soil, there 
is, as represented in fig. 29, in many parts only one layer, not 
composed of cells, but of a nucleated material which is not 
unlike the coelenchyme of Stratiodrilus, doing duty both 
as the myocytes of the longitudinal muscular fibres and also 
as the somatic ccelomic layer. But in some parts this material 
is divided into two layers — an outer, surrounding the inner 
edges of the muscular fibres, and containing nuclei situated 
in close contact with the latter, and an inner, provided with 
nuclei flattened tangentially, situated at irregular intervals. 
The substauce of these two layers is in great measure in 
continuity, the filaments of the protoplasm being traceable 
from one to the other, and in some parts the coalescence 
appears to be complete, so that only one layer is recognisable. 
In this Encytr^eid, then, the relations of the somatic 
peritoneal layer are either more primitive than in the 
Oligochaeta in general, or have become secondarily 
modified, and are closely comparable to what occurs in 
Protodrilus, approximating to a certain extent towards 
the condition prevailing in the Histriobdellidae (fig. 28). 
It seems to me highly probable that in this respect, as in the 
relations of the nervous system to the epidermis,^ the more 
primitive, or simplified, condition may prove to be by no 
means exceptional among the smaller and simpler Chaetopoda. 
Jaws and Digestive System. 
The jaw - apparatus of the Histr iobd elli dae is an 
extremely complex structure. Altogether some thirty distinct 
^ See E. S. Goodrich, “ On the Structure and Affinities of Sacco- 
cirrus,” ‘Quart. Journ. Micr. Sci.,’ N.S., vol. 44, p. 422. 
