1895.] HIDBAOHNID FOTTITD IN" COBNWALL. 179 



ostracum," is the living layer or hypodesm. It is in the cells of this 

 layer that the chitin is deposited which forms the chitinous plates 

 of the cuticle. The cells in which the chitin is deposited occasionally 

 increase greatly in size, the swelling being inward ; so that at the 

 enlarged point the chitin projects further into the body than the 

 other portions of the cuticle (fig. 24, cp., the hind end of the first 

 plate), and the larger plates are so abundantly pierced by large irre- 

 gular holes or areolations that in section the chitin often looks like 

 detached rods, or laminae. The chitin is often thinner at the edge 

 of the plate than elsewhere, and in that case gradually diminishes 

 to an edge at the periphery, so as to present a knife-edge section. 

 The chitin here is pierced only by smaller pores, not by the large 

 areolations. The cells of the hypoderm generally send proto- 

 plasmic tongues into the areolations, often entirely filling them. 

 Above the chitinous plates the epiostracum often persists, as 

 observed by Schaub in ffi/drodroma : when it does so it most 

 usually dries up and becomes a very thin layer of dead flattened 

 cells ; but the compression or crumpling of the convex outer side 

 produces a greater thickness or opacity in the middle of the cell, 

 which gives the plates a somewhat spotted appearance over the 

 areolations. Most commonly, however, particularly on the dorsal 

 surface, the epiostracum not only dries up but rubs off and is 

 entirely lost ; the ectostracum also, in the same cases, dries up 

 above the plate and becomes an extremely thin layer ; so that the 

 two outer layers of the cuticle over the plate are not nearly so 

 thick as in other situations ; hence the plates of the dorsal surface 

 lie at the bottom of shallow depressions ; this applies to the larger 

 areolated plates only, not to the small hair-bearing plates. 



The larger plates give attachment on their inner sides to the 

 dorso-ventral and other muscles. 



The Dermal Glands (Plate IX. figs. 23, 24, 25, 26). 



The general arrangement, comparative size, and position on the 

 dorsal surface of these glands, which are so well-knowTi in the 

 Hydrachnidse, is very similar to that described by Schaub in 

 Hydrodroma and by Haller ^ ; the glands are not, however, so 

 sti'ictly confined to the dorsal surface as they seem to be in 

 Hijdrodroma ; there are some on the edge of the ventral surface 

 near the anterior and posterior ends of the body. The glands 

 themselves differ considerably from those described by Schaub, 

 inasmuch as they are entirely without the chitinous external coating 

 and the chitinous network of strengthening ribs which that author 

 found ; they are enveloped simply by a soft membranous tunic, 

 and are formed of large, delicate, very loose cells, in which a 

 nucleus is rarely to be detected : these cells stain but slightly, and 

 the greater number are usually found to have broken down, either 

 during the life of the specimen or during its preparation. There 



1 "DieArten und Gattungen der Sohweizer Hydrachnidenfauna," Mittbeil. 

 der Schweizer entom. G-esellsch. 1882, p. 18. 



12* 



