19 
layer of air appears to l)e pressed into the cavity in whicli the inner gills lie. The outer gills thus 
possess an active as well as a j^assive function with respect to the inner gills. 
It is possible that the outer gills, in addition to performing these fimctions, constitute in 
themselves organs of respiration. The blood circulates between the two walls of the gills, and 
whether respiration takes place depends upon the penetrability of the walls to the gases to he ex- 
changed. The ventral wall of chitine is thick — thicker than the chitinous wall in some other 
regions of the body, as, for examjile, the thoracic legs (in which also the blood circulates). The 
dorsal wall is, on the other hand, moderately thin. It is, moreover, exposed to the moist air in 
the chamber in which the inner gills lie. It would seem probable, therefore, that to some extent 
respiration may take place through the dorsal Avail. 
I conclude, then, that the function of the last three pairs of outer gills is chiedy a relative 
one, namely, that of protecting the inner gills from dessiccation and of supplying air to them ; but 
that they also act independently, as organs of respiration, though limited in this function. 
The Inner Gills. The general external features of the inner gills of PorceUio have been 
studied by several authors : first by Tekvikanus and later by Dua^ernoy, Lereboullet and Leydig, 
The last named author also investigated their internal structure in the species PorceWio armadiUoides.^ 
I find that the figure he gives for this species applies also in general to PorceUio scaher. (Full 
notice of Leydig’s account of the histology will be given heloAv under Gi/listicus convexicus.) 
A cross-section of the inner gill of PorceUio scaber presents the appearance shown in Fig. 8. 
It is seen to consist of a flattened sac the cavity of which contains blood. The wall of the sacF 
is composed of an outer thin layer of chitine and an inner thick nucleated layer — the hypoderm. 
The hypodermic layers on the tAvo opposite dorsal and ventral sides of the gill are connected by 
pillars. The lines representing the structural elements of these pillars are seen to be continuous 
at each end Avith the structural elements of the hypodermic layer. 
Examined as to its mimite anatomy the hypoderm is seen to consist of a continuous layer 
of granular and striated matter. In other words there are no cell-boundaries, the cell structure 
being indicated alone by the nuclei. The striae of the protojAlasm are arranged at right angles to 
the surface of the gill. The nuclei are large and in general ovoidal in shape. They shoAV a definite 
arrangment, — the long axis of the ovoid being at right angles to the face of the gill and the 
narroAV end on the side toAvard the cavity of the gill. Tlie granules of chromatiiie of the nuclei 
also conform to a laAv of arrangment, namely, they tend to he aggregated at the narroAV end of 
the nucleus, toward the cavity of the gill. It Avould appear that these structural features have a 
significance in reference to the passage of the gases concerned in respiration through the hypoder- 
mic layer. 
The hypoderm at its inner margin is bounded by a clearly defined line Avhich is immedi- 
ately in contact Avith the blood Avithin. The appearance of this line is such as to suggest that it is 
the dense (chitinized ?) inner surface layer of the hypodermic tissue. Whether this is the case or 
Avhether the line represents a remnant of a mesodermic layer hounding the blood cavity, I have not 
had an opportunity, through the study of the embryology of the gill, to ascertain. 
Physiology of the Inner Gills. The function of the inner gills is that of respiration 
in a medium of moist air. That the animals recpiire air charged with moisture as a condition for 
^ Concerning tlie identity of this species, see p. 20, 
