— 25 — 



This appearance lasts but a short time. The waves gradually become flatter and narrowei- 

 and in the course of 10 minutes pass away altogetlier. 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 back and in about 30 minutes is fuUy regained. 



Minute Extern al Anatomy. Under liigh magnification the chitinous wall shows 

 superficial markings forining a patteru-work similar to that ah-eady described for Porcellio scaher. 

 The dots, as in the species ah-eady described are the nuclei of the hypodennic layer. The piUars 

 of the hypoderni come into view by focussing at difi'erent 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 Porcellio 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 

 oviter 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 modiüed for respiration and containing in the 

 living animal, as noted above, aii-. 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 clütine 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 hypoderm 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 tlie 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 

 hypoderm 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 acconnt the chitinous wall of this part of the gill has, as noted above, 

 an undulated fonn. 



The structure and relations of the general part of the gill. In Onisciis the 

 general cavity of the gill is not a simple cavity as in Porcellio but is separated into a number of 

 Chambers by means of pai-titions lying parallel to the walls of the gill. 



In the general part of tlie 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. Fach charaber is bounded 

 above and below by a thin wall. It results from this that the cavities of the two Chambers are 

 separated by a double wall (Pt.). This appears as two fine parallel lines between which is a very 

 narrow space. Lying in this space and occurriug at frequent intervals are elongated nuclei. The 

 outer walls of each Chamber lie immediately within the hypodermic layers on the respective dorsal 



Bibliotheca Eoologica. Heft 36. ^ 



