394 THOS. H. MONTGOMERY jr., 
and lateral regions of the head (Figs. 1, 3, 4, 6, Pl. 37; Fig. 17, Pl. 38), 
where it constitutes a much modified sensory epithelium, and also 
where it forms the anterior wall of the eye (E. Hyp Figs. 1, 3, 6); 
these regions will be described under the headings “Nervous system” 
and “Eye”. This thickening on the ventral surface of the head be- 
comes lower behind the head, and in the trunk region posteriorly to 
about the plane of the atrium the hypodermis is much more flattened. 
But through this extent of the trunk, while it becomes very much 
flattened on the lateral aspects of the body, it is elevated into a ridge 
in the medio-ventral line, just at the point of attachment of the neural 
lamella (Figs. 13, 14, Pl. 37; Fig. 17, Pl. 38); and another thickening 
occurs in the medio-dorsal line, this dorsal thickening being broader but 
not so high as the ventral ridge (Figs. 13, 14, 16, Pl. 38; Figs. 61, 64, 
Pl. 40; Fig. 65, Pl. 41). Behind the plane of the atrium (Fig. 74, 
Pl. 41) in all the region of the cloaca (Fig. 72) and the tail lobes 
(Figs. 71, 73), the hypodermis becomes higher, so that both in the 
mid line and the lateral aspects it has the same thickness; con- 
sequently, in passing backwards from the plane of the atrium one 
finds that the medio-dorsal and medio-ventral thickenings disappear. 
The latter thickening diminishes in height gradually from the head 
region backwards. ; 
The cellular components of the hypodermis are: large cells which 
are the producers of the cuticula, and may be termed the cuticular 
cells; gland cells; and nervous elements, which occur in 
the form of nerve cells, of terminations of the fibres of the chromophilic 
nerve cells of the central system, and of sensory cells. All these 
nervous elements will be described under the heading “Nervous 
system”. 
The cuticular cells extend from the cuticula inward to the 
muscular layer (or to the parenchym where that layer is lacking), 
and form a layer one cell thick (Hyp Figs. 13, 14, Pl. 37; Figs. 15—17, 
22, 24, 25, Pl. 38, Figs. 27, 28, Pl. 39; and other figures). In the head 
region these are exceedingly elongated (Hyp Figs. 1, 6, Pl. 37), and to a 
eradually diminishing degree in the medio-ventral line of the trunk 
also (Fig. 13, Pl. 37; Figs. 15, 17, Pl. 38); such cells represent a 
particularly modified sensory type of the cuticular cells, and they are 
characterized in addition to their length by their being clearly separated 
from one another with narrow intervening spaces. Everywhere else 
in the trunk region as far back as the plane of the atrium, that is 
everywhere dorsally and laterally, these cuticular cells are more 
