The Structure and Development of the Segmental Excretory Organs of Asellus aquations (Linné)
Authors: Needham A. E. 
Published: April 2021

The maxillary organ (‘gland’) of Asellus is pyramidal in form and consists of an end sac in the form of an inverted V placed transversely to the body, a duct describing four vertical loops, one anterior and three posterior to the end sac, and a chitin-lined exit-tube leading from the duct to the exterior and enlarging along its course into a bladder. A sphincter consisting of four cells with two sets of intracellular fibrils controls the aperture from end sac into duct. The external aperture is on the medio-posterior face of the limb-base. The organ is anchored to various points on the exoskeleton by connective tissue strands. The spaces among the coils are partly filled by cells which accumulate reserve materials but leave channels for the circulation of blood. Histologically the organ resembles those of other Crustacea in all essentials. The exit-tube is ectodermal, the rest of the organ mesodermal. Duct and end sac originate from independent rudiments, the former from a single large cell (? nephroblast) located where the medial wall of the maxilla joins the body, the end sac from a number of smaller, scattered cells dorso-laterally to the duct rudiment. Duct and end sac are probably derived from the second and third mesodermal teloblasts respectively. The sphincter originates from the end sac. Cells of the genital rudiment are associated with the end sac in early stages. There is considerable evidence that the segmental excretory organs are homologous throughout the Crustacea. They differ from the coelomoducts of other Arthropods and of other phyla in having a composite structure. This suggests that the duct may be homologous with the nephridia of other groups of animals. There is functional and morphological evidence for this but nephridia are typically ectodermal in origin, so that if they are homologous there must have been a movement of the presumptive nephridioderm, during the evolution of the Crustacea, from the ectoderm to the mesoderm. The antennal organ of Asellus (Néméc’s gland) consists of a compact group of five large cells containing intracellular vacuoles. It is suspended in the body cavity by connective tissue strands, and has no external opening. A nerve supplies it from the antennal ganglion. In the adult it appears to be an organ of internal secretion: the vacuoles have been seen in the distended and in the exhausted phases. Néméc’s gland originates close to the epidermis at the antennal-mandibular intersegment. It appears to be mesodermal and probably represents the duct of the typical segmental organ (not the end sac). Ter-Poghossian’s organ, consisting of a large number of small cells in a compact group attached to the ventral epidermis, median and anterior to Néméc’s gland, has no other anatomical relations. It appears to be ectodermal in origin. The ‘rosette’ glands resemble structurally the tegumental glands of Decapods. The gland cells appear to be mesodermal, but the origin of the central duct cell was not determined. Gastrulation in Asellus is essentially similar to the process in other Malacostraca, but it seems possible that the distinction between naupliar and post-naupliar regions is not so sharp as has usually been maintained. The value of the embryological evidence concerning the homology of the parts of the second maxilla is considered. It does not entirely agree with interpretations based on adult morphology.