FIG. 73.—_Inachus mauritanicus_, × 1. =A=, Low male; =B=, middle male; =C=, high male; the great chela of the right side is the only appendage represented. ] =Partial and Temporary Hermaphroditism. High and Low Dimorphism.= The reproductive phases of animals are frequently rhythmic, periods of growth alternating with periods of reproduction. This is well exemplified in the case of the ordinary males of _Inachus mauritanicus_, of some other Oxyrhynchous crabs, and of the Crayfish _Cambarus_.[77] During the breeding season the males of _I. mauritanicus_ fall into three chief categories: Small males with swollen chelae (Fig. 73, A), middle-sized males with flattened chelae (B), and large males with enormously swollen chelae (C). On dissecting specimens of the first and third categories it is found that the testes occupy a large part of the thoracic cavity and are full of spermatozoa, while in the middle-sized males with female-like chelae the testes appear shrivelled and contain few spermatozoa. These non-breeding crabs are, in fact, undergoing a period of active growth and sexual suppression before attaining the final state of development exhibited by the large breeding males. This phenomenon is obviously parallel to the “high and low dimorphism”[78] so common in Lamellicorn beetles, where the males of many species are divided into two chief categories, viz. “low males” of small size in which the secondary sexual characters are poorly developed, and “high males” of large size in which these characters are proportionately much more highly developed than in the low males. The only difference between the two cases is that whereas in the beetles growth ceases on the attainment of maturity in the low degree, in the Crustacea the low male passes through a period of growth and sexual suppression to reach the high degree of development. The condition of the middle-sized males may be looked upon as one of partial hermaphroditism, indications of the female state being found in the flattened chelae and in the reduced state of the testes. This interpretation is greatly strengthened by the state of affairs observed in the life-history of the male Sand-hoppers, Amphipods of the genus _Orchestia_.[79] In the young males of several species of this genus, at the time of year when they are not actively breeding, small ova are developed in the upper part of the testes of more than half of the male individuals, these ova being broken down and reabsorbed as the breeding season reaches its height. Nor is this phenomenon confined to this genus; in the males of a number of widely different Crustacea these small ova are found in the testes at certain periods of the life-history (_e.g._ _Astacus_[80]), when the animal is not breeding.