OCR Text |
Show 1895.] HYDEACHNID EOUND IN COBNWALL. 179 ostracum," is the living layer or hypoderm. 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 irregular 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 protoplasmic tongues into the areolations, often entirely filling them. Above the chitinous plates the epiostracum often persists, as observed by Schaub in Hydrodroma : 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 tbe 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 aud other muscles. The Dermal Glands (Plate IX. figs. 23, 24, 25, 26). The general arrangement, comparative size, aud position on the dorsal surface of these glands, which are so well-known in the Hydrachuidae, is very similar to that described by Schaub in Hydrodroma and by HallerT; the glands are not, however, so strictly confined to the dorsal surface as they seem to be in Hydrodroma ; 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 Schweizer Hydrachnidenfauna," Mittheil. der Schweizer entom. Gesellsch. 1882, p. 18. 12* |