25 
This appearance lasts but a short time. The waves gradually become flatter and narrower 
and in the course of 10 minutes i)ass away altogether. The shining white aspect gives place to 
the ordinary appearance of the rest of the gill. If the animal is then restored to air the original 
appearance gradually comes Imck and in al)Out 30 minutes is fully regained. 
Minute External Anatomy. Under liigh magnification the chitinous wall shows 
superficial markings forming a pattern-work similar to that already described for PorceUio scaher. 
The dots, as in the sjjecies already described are the nuclei of the hypodermic layer. The pillars 
of the hypoderm come into view by focussing at different levels. Along the margins of the blood- 
channels these pillars are arranged in rows, forming a kind of broken wall to the blood-channels. 
Within the cavity of the gill blood-corpuscles may be seen. They consist of the two forms 
of cells already described for PorceUio scaher. 
Internal Anatomy. Fig. 12 represents a cross-section of the gill taken in the position 
of the line ss'. Fig. 11. It is seen that the gill is of moderate thickness throughout and that the 
outer fourth (right side of the figure) is much thinner than the rest. This thin portion of the gill 
in the special part; that is to say, the part specially modified for respiration and containing in the 
living animal, as noted above, air. The remaining thicker portion constitutes what we have desig- 
nated above the general part of the gill. 
The chitinous wall of the gill varies in thickness in different regions. It shows its greatest 
development on the dorsal side of the general part, where the chitine makes up nearly one-third 
of the breadth of the gill. On the ventral side it is everywhere thin, but less so in the general 
than in the special part. In this latter part the chitinous wall is thrown into elevations and de- 
pressions (see also Fig. 13). 
The liypoderm consists of a continuous layer of tissue lying immediately within the chitine 
and, in addition, the bridges or pillars of tissue referred to above. In general the hypoderm layer 
varies in thickness with the chitine. The nuclei are conspicuous and vary widely in respect to size, 
form and position. Where the layer is thick the nuclei are large and tend to be elongated in form 
and arranged with the long axis at right angles to the wall of the gill. Where the layer is thin 
the nuclei are flattened and lie parallel with the wall of the gill. In the special portion of the gill 
where the hypodermic layer is very thin the nuclei appear to be withdrawn from the layer and to 
be associated with the tissue of the pillars (Fig. 13, Hy. pL). It is seen further that the layers of 
hypodei’m between the bases of the pillars are arched outwards. In other words, where the pillars 
occur the opposite dorsal and ventral hypodermic layers are drawn inward, thus being thrown into 
the form of waves. On this account the chitinous wall of this part of the gill has, as noted above, 
an undulated form. 
The structure and relations of the general part of the gill. In Oniscus the 
general cavity of the gill is not a simple cavity as in PorceUio but is separated into a number of 
chambers by means of partitions lying parallel to the walls of the gill. 
In the general part of the gill there are two of these chambers, an inner or dorsal chamber 
and an outer or ventral one (Fig. 12, d.b.c., v.b.c.). They contain blood. Each chamber is bounded 
above and below by a thin wall. It results from this that the cavities of the two chandlers are 
separated by a double wall (Ft.). This appears as two fine parallel lines between which is a very 
narrow space. Lying in this space and occurring at frequent intervals are elongated nuclei. The 
outer walls of each chamber lie immediately within the hypodermic layers on the respective dorsal 
Bibliotheca Koologica. Heft 25. 4 
