— 13 — 



intersection of which forms the marginal angle referred to above. The surface of this region pre- 

 sents the appeai'ance 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 chitinous wall of the gill and contain air. 



Minute Anatomy. Examined with a high power of the microscojje, the several parts 

 just noticed present the following features: 



The chitinous wall of the general i)art of the gill sliows superficial markings, consisting of 

 niiniite pits which ajipear as dark points. They are arranged so as to form a pattern composed 

 of polyginal areas (Fig. 2). 



The bodies referred to above as dots are seen to present the appearance of nuclei. They 

 lie immediately under the chitine and aie of irregulär sige and form. By focussing at different 

 levels it may be seen that associated ynth many of the nuclei are Strands of tissue which extend 

 from the dorsal to the ventral wall 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-work the chitine is sculptured in a peculiar way. The surface in thrown into irregulär 

 polyginal areas which are separated l)y furrows (Fig. 3b; P. a, Gr.). The areas themselves are 

 sculptured by luuch finer furrows, lying in short angular lines. The elevations and depressions thus 

 formed give the surface of each area the appearance of a pattern. 



That the larger furrows or grooves separating the polyginal areas contain air may be de- 

 monstrated by mounting a gill in water, when they are seen to appear white by reflected and dark 

 by transmitted light. If a gill is placed in alcohol before examining the net-work 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. When 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 the blood. They are of two kinds, namely: spherical, nucleated 

 cells and larger, highly granulär, nucleated cells of irregulär form. The former which are much 

 more numerous than the latter are the ordinary blood corpuscles; the latter are leucocytes. 



The two forms of blood corpuscles may be seen elsewhere in the gill but occurring sparsely 

 as compared with the special region above mentioned. It would appear that Wagner (see reference 

 p. 10) in referring to the "gouttelettes de la gi-aisse" and in stating that they occui- aggregated 

 chiefly in one place in the gill had observed the corpuscles distributed as above described. 



A portion of the marginal region of the white body presents an appearance somewhat re- 

 sembling a piece of branching coral (Fig. 5). The whole structure appears to be hoUow, with smooth 

 thin waUs. The appearance of irregidar markings on the walls is probably due to shrinkage, con- 

 sequent upon the passing out of air. 



Internal Anatomy. Fig. 4 rejiresents a cross-section of the gill taken in the direction 

 of the Hne 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 



