Visceral Anatomy of Gammarus lacustris lacustris Sars (Crustacea: Amphipoda)

A study was made of the alimentary tract, reproductive, excretory, nervous, and circulatory systems of the western North American amphipod, Gammarus lacustris lacustris Sars. Where information concerning individual organ systems was available, the anatomy of G. l. lacustris was compared with isopods, several European amphipod species, and decapods. The alimentary tract consists of three main divisions: fore-gut, midgut, and hind-gut. Seven caeca arise from the mid-gut. The dorsal median caecum and two pairs of hepatopancreatic caeca join the midgut near its anterior end. A pair of rectal caeca join the mid-gut near its posterior end. A complex gastric mill, consisting of plates, ossicles, and folds, is the most conspicuous aspect of the cardiac stomach. The pyloric stomach is characterized by a complex filter apparatus. The histological structure of the fore-gut and hind-gut walls consists of a chitinous intima, columnar epithelium, tunica propria, circular and sometimes longitudinal muscle, and a "serosa." The mid-gut wall is composed of an epithelial layer, a basement membrane, a tunica propria, circular muscle, and a "serosa." The columnar cells of the dorsal median caecum and rectal caeca are non-vacuolate and basophilic, and are thought to function in salt resorption. The columnar cells of the hepatopancreatic caeca are mostly vacuolated and function in enzyme secretion and absorption. The function of the tegumental glands remains unknown. The testes are slender, paired organs, expanding posteriorly into seminal vesicles. The vasa deferentia open at the base of the seventh pereionic segment. The histological structure of the male system is thought to reflect a resting condition in the specimens studied. The ovaries are elongate strands of oocytes surrounded by oogonia. The paired oviducts open at the base of the fifth pereionic segment. The antennal glands consist of an end-sac, an excretory duct, glandular tissue, and an exit duct, which opens on the lower distal portion of the first antennal segment. The excretory duct is a syncytium, and like the glandular tissue, probably functions in resorption, although the cells of the latter are probably secretory as well. The functions of the end-sac and sphincter are problematical. The nervous system consists of a brain, subesophageal ganglia, and paired pereionic and pleonic ganglia connected by paired longitudinal commissures, all of which give rise to nerve branches. There are no horizontal commissures, a