A Comparative Study of the Statocysts of Eumalacostraca. with special reference to the Macrura

Abstract

Summary. In Anaspides the statocyst is situated in the antennulary basal segment; the sac is boot‐shaped, with a narrow slit‐like opening antero‐medially; about 35 jointed club‐shaped hairs are suspended in a single series from the roof and near the opening of the sac; each hair contains one to two small bodies, presumably the statoliths. Tattersall's discovery of what is probably a very rudimentary form of antennulary statocyst, without static hairs or statolith, has been confirmed in Hansenomysis. No trace of antennulary statocyst has been found in Cumacea, Tanaidacea, Euphausiacea, Isopoda, and Amphipoda; although tactile hairs are often present on the basal segment, they do not represent the rudimentary static hairs, since they are usually present in Macrura (e. g., Crangon, Caridina), in addition to the statocyst. The cephalic statocysts of Gamrnarus chevreuxi have been described. In the Macrura examined there is a definite relationship between the stylocerite and the statocyst. When the inner fold of the stylocerite fully or partially overlies the basal segment, a well developed statocyst, as a rule, is present; when the stylocerite is lateral to the segment a comparatively simple statocyst (e. g., Crangon) or a very rudimentary form (e. g., Caridina) may be present, but generally it is absent (e. g., Hippolyte). Thus, within the Macrwa, the evolution of the statocysts is correlated with the character of the stylocerite. In Palzmonetes and Leander the statocyst ia highly developed; the sac is peach‐like, with a dorsal triangular opening overlapped by the stylocerite; the sensory cushion is on the floor of the sac, its longer axis being parallel (Palzmonetes) or transverse (Leander) to that of the sac; there are 56 typical plumose static hairs in Palzmonetes, 85 in Leander. The statoliths are of the usual type, namely, grains of sand taken in after each moult. In Alpheus the sac is large, with an oval aperture directed laterally and partially overlapped by the stylocerite; a series of eight bristles overlap and protect an emargination in the dorsal wall of the opening; the sensory cushion is inconspicuous, on the almost vertical floor of the sac; there are about 65 typical static hairs. Besides the static hairs, the statocyst of Alpheus has a great number of tactile hairs scattered on the outside of the sensory cushion. These may possibly be folded into the cavity during the formation of the statocyst. The statocyst of Ogyrides is almost identical with that of Alphew, and supports the view that this genus belongs to the family Alpheida. In Crangon vulgapis the stylocerite is lateral to the statocyst, the opening of which is protected by a row of 25 to 27 large bristles; the 30 static hairs axe arranged in series on an undulating sensory ridge on the lateral wall of the sac. The most rudimentary form of statocyst that has hitherto been known to occw in Decapoda is present in Caridina and Xiphocaris. It is just an elongate shallow depression on the dorsal surface of the antennulary basal segment, and contains neither static hairs nor statoliths. I have not dealt with the detailed histology of the muscular and nervous systems, although they are figured in detail. For full description reference should be made to Prentiss (1901). The sensory ganglion cells are all typically bipolar and elongate. They are situated at some distance from the bases of the static hairs, each of which they supply with a single fibre from a single cell (the peripheral fibre enters only the spherical enlargement of the hair‐base). They may be regarded as equivalent to neurons between the peripheral and central fibres (PL II. fig. 12: PL III. figs. 25, 27; P1. IV. fig. 31; PI. V. figs. 36, 37; P1. IX. fig. 62).