THE DEPOSITION OF SKELETAL STRUCTURES IN THE CRUSTACEA. I. THE HISTOLOGY OF THE GASTROLITH SKELETAL TISSUE COMPLEX AND THE GASTROLITH IN THE CRAYFISH, ORCONECTES (CAMBARUS) VIRILIS HAGEN—DECAPODA (1960)

1. The gastrolith discs of the fresh-water crayfish, Orconectes virilis Hagen, are located in the anterior lateral walls of the cardiac stomach and are themselves modified portions of the stomach wall.2. Histologically, the cuticular surface of the completed gastrolith disc (Stage C4), which forms part of the lining of the stomach, is composed of three differentiated layers: a thin non-calcified epicuticle, formed before molt; a thicker noncalcified exocuticle, laminated in nature, but neither crossed by pore canals, nor tegumental ducts, formed following molt and completed during Stage B; and the endocuticle, the thickest of the three layers, characterized by neither being laminated nor crossed by pore canals and possessing round granules without structure, embedded in a loose fibrous meshwork. The largest of these granules are located in the outer layers of the endocuticle very close to the exocuticle. The tissue complex of the completed gastrolith disc is composed of a single layer of columnar epidermal cells, a loose "spongy" type of sub-epidermal connective tissue, composed of cells of Leydig and a number of the transient reserve cells which are also observed in the epidermis. The tissue complex is well supplied by branches of the antennary arteries and has a large blood-cell-forming gland.3. Marked histological changes are observed during Stage D when premolting processes, associated with gastrolith deposition, are set into motion. The epidermis increases greatly in height, invaginates, and begins to elaborate the matrix of the gastrolith which is filled at its lateral edges with granules like those observed in the C4 endocuticle. At this same time the active epidermal cells take on a branched attenuated appearance at their apical ends, and many intercellular spaces become apparent between the cells. From Stage D0 through most of Stage D4 continued synthesis, elaboration, and calcification of the gastrolith matrix occur, its thickness increasing from around 300 µ (D0) to 3-4 mm. (D4). Concomitant with these changes is an increase in the number of reserve cells in both epidermis and connective tissue and a stretching, and thus compression, of the sub-epidermal connective tissues, the latter process being correlated with the increasing size of the forming gastrolith. Near the end of Stage D (D4), the epidermis has completed its task of depositing the gastrolith, retracts from it, undergoes rapid growth accompained by much folding and achieves its greatest height. It then begins to elaborate the epicuticle, the only pre-exuvial skeletal component deposited in this site. When this task is achieved by the epidermis, correlated with the many tasks it performs in other skeletal areas, molting ensues and the old stomach lining and the formed gastroliths are released into the stomach.4. Postmolt histological changes in the gastrolith disc are associated with continued synthesis and elaboration of cuticular components. The "intermolt condition" is achieved in Stage C4, when these synthetic tasks are complete, and from this stage the entire histological cycle repeats itself.