13 
intersection of wliicli forms the marginal angle referred to above. The surface of this region pre- 
sents the appearance of a net-work. The meshes of the net-work appear as dark lines under trans- 
mitted light, indicating that they are furrows in the chitinoiis Avail of the gill and contain air. 
Minute Anatomy. Examined Avith a high poAver of the microscoije, the several parts 
just noticed present the following features : 
The chitinous Avail of the general gill sliows superficial markings, consisting of 
minute pits which appear as dark ]joints. They are arranged so as to form a pattern composed 
of polyginal areas (Eig. 2). 
The bodies referred to above as dots are seen to present the appearance of nuclei. They 
lie immediately under the chitine and are of irregular sige and form. By focussing at different 
levels it may be seen that associated Avith many of the nuclei are strands of tissue which extend 
from the dorsal to the ventral Avail of the gill. These form pillars and will be further described below. 
In that portion of the special region of the gill where the wall presents the appearance 
of a net-Avork the chitine is sculptured in a peculiar way. The surface in throAvn into irregular 
polyginal areas which are separated by furrows (Fig. 3b; P. a, Gr.). The areas themselves are 
sculptured by much finer furroAvs, lying in short angular lines. The elevations and tlepressions thus 
formed give the surface of each area the apjAearance of a pattern. 
That the larger furrows or grooves separating the polyginal areas contain air may be de- 
monstrated by mounting a gill in Avater, when they are seen to appear Avhite by reflected and dark 
by transmitted light. If a gill is placed in alcohol before examining the net-Avork does not appear 
dark under transmitted light. The finer furrows of the polyginal areas do not appear to hold air. 
Underneath the chitine presenting these special modifications may be seen corpuscles which 
here occur somewhat in accumulation. Wlien pressure is applied to the cover-glass these corpuscles 
move independently. Examination of a drop of blood taken from the body of a living animal shows 
the same corpuscles present in tlie blood. They are of two kinds, namely: spherical, nucleated 
cells and larger, highly granular, nucleated cells of irregular form. The former which are much 
more numerous than the latter are the ordinary blood corpuscles; the latter are leucocytes. 
The tAvo forms of blood corpuscles may be seen elseAvhere in the gill but occurring sparsely 
as compared with the special region above mentioned. It Avould ajApear that Wagner (see reference 
p. 10) in referring to the “gouttelettes de la graisse“ and in stating that they occur aggregated 
chiefly in one place in the gill had observed the corpuscles distributed as above described. 
A portion of the marginal region of the Avhite body presents an appearance somewhat re- 
sembling a piece of branching coral (Fig. 5). The Avhole structure appears to be hollow, with smooth 
thin Avails. The appearance of irregular markings on the walls is probably due to shrinkage, con- 
sequent upon the passing out of air. 
Internal Anatomy. Fig. 4 represents a cross-section of the gill taken in the direction 
of the line s s' Fig. 1. The gill is seen to fall into two portions, a thinner and a thicker; these 
correspond, respectively, to the general and special parts, described above. The chitinous wall (Ch) 
is in general thicker on the- lower or ventral side than on the upper or dorsal side. Within the 
chitine is a layer of tissue with conspicuous nuclei — the hypoderm (Hy). The hypodermic layers 
lining the two opposite chitinous walls are not entirely separated but, in the general portion of the 
gill, are connected by extensions, forming pillars (PL). The space crossed by these pillars is the 
