/'•- 



— 19 — 



layei- of air appears to be pressed into the cavity in whicL the inner gills lie. Tlie outer gills thus 

 possess an active as well as a passive fiinction witli respect to the inner gills. 



It is possible tbat the outer gills, in addition to performing tbese functions, constitute in 

 tbemselves organs of respii'ation. Tbe blood cü'culates between the two walls of the gills, and 

 whether respiration takes place depends upon the penetrability of the walls to the gases to be ex- 

 changed. The ventral wall of chitine is thick — tliicker tlian the chitiuous wall in some other 

 regions of the body, as, for example, the thoracic legs (in which also the blood circulates). The 

 dorsal wall is, on the other band, inoderately thin. It is, nioreover, exposed to the moist air in 

 the Chamber in which the inner gills lie. It would seem probable, therefore, that to some extent 

 respiration niay take place through the dorsal wall. 



I conclude, then, that the function of the last three paii'S of outer gills is chiefly a relative 

 one, namely, that of protecting the inner gills froin dessiccation and of supplying aii' to them ; but 

 that they also act independently, as organs <jf respiration, thoiigh limited in this function. 



The Inner Gills. The general externa! features of the inner gills oi Forcellio have been 

 studied by several authors : first by Teeviranus and later by Duvebnoy, Lekeboullet and Leydig, 

 The last named author also investigated their internal structure in the species PoireUio arniadilhides.^ 

 I find that the figure he gives for this species applies also in general to Porcellio scahtr. (Füll 

 notice of Leydig's account of the histology will be given Ijelow under Gylisticus convexicus.) 



A cross-section of the inner gill of Forcellio seaber presents the appearance shown in Fig. 8. 

 It is Seen to consist of a Hattened sac the cavity of which contains blood. The wall of the sack 

 is coniposed of an outer thin layer of chitine and an inner thick nucleated layer — the hypoderm. 

 The hypodermic layers on the two opposite dorsal and ventral sides of the gill are connected by 

 pillars. The lines representing the structural Clements of these pillars are seen to be continuous 

 at each end with the structiu'al Clements of the hypodermic layer. 



Examined as to its minute anatomy the hypoderm is seen to consist of a continuous layer 

 of granulär and striated matter. In other words there are no cell-boundaries, the cell structm-e 

 being indicated alone by the nuclei. The Striae of the protoplasm are arranged at right angles to 

 the surface of the gill. The nuclei are large and in general ovoidal in shape. They show a definite 

 arrangment, — the long axis of the ovoid being at right angles to the face of the gill and the 

 narrow end on the side toward the cavity of the gill. The granules of chromatine of the nuclei 

 also conform to a law of arrangment, namely, they tend to be aggregated at the narrow end of 

 the nucleus, toward the cavity of the gill. It would 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 which is immedi- 

 ately in contact with the blood mthin. 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 

 whether tlie line represents a remnant of a mesodermic layer bounding the blood cavity, I have not 

 had an opportunity, through the study of the embryology of the gill, to ascertaiu. 



Physiology of the Inner Gills. The function of the inner gills is that of respiration 

 in a medium of moist air. That the animals retiuire air charged with moisture as a condition für 



\ 



' Concerning tbe identity of this species, see p. 20. 



