834 FRESH-WATER BIOLOGY 



these organs are softer, the horny tips are undeveloped, and the 

 copulatory hooks on the ischiopodites of the peraeopods are small. 



According to the general rule, that the males assume the first 

 form in autumn, the copulating season falls in the autumn, and 

 copulation may be repeated in the winter months. The male 

 seizes the female and holds it, sternites against sternites, chiefly 

 by the aid of the hooks of the ischiopodites of the peraeopods. 

 The sperm is discharged and stored in the female's annulus ven- 

 tralis, a pocket on the thoracic sternum, which thus serves as 

 receptaculum seminis. Oviposition takes place later, generally in 

 spring. 



This seasonal cycle, as described, is not observed in all species, 

 but there are some, in which the alternation of the two forms of 

 the male is irregular and not connected with the seasons, and 

 where copulation and oviposition are also irregular. It has been 

 found that regularity of the annual cycle is connected with a habi- 

 tat in water which is subject to regular and considerable seasonal 

 changes of temperature (species living in rivers and ponds), while 

 irregularity of the life-cycle is found among those which live * pref- 

 erably in water with slight temperature changes and that at the 

 same time is rather cool (species of mountain streams and of cool 

 springs or groundwater) . 



The fresh-water Malacostraca depend entirely upon the presence 

 of water, and cannot leave the water as a rule. This holds good 

 for the Isopoda, Amphipoda, and Mysidacea, and also for the 

 Atyidae and Palaemonidae among the Decapoda. In the water, the 

 Isopods (except the parasitic forms) crawl around on the bottom, 

 under stones, or climb among water weeds, but do not move by 

 swimming. The Amphipods are very lively in their movements, 

 which consist chiefly of swimming, often lying upon the side. The 

 swimming is often done in jerks, by curving and stretching the 

 compressed body. They move also by climbing among water weeds, 

 but hardly ever by crawling. All Mysidacea are distinctly swim- 

 ming forms, and so are the Atyidae and Palaemonidae among the 

 Decapods, while the movements of the crayfishes are of various 

 kinds, but fall under two main heads: crawling and swimming. 

 The first is the general mode of locomotion. It is not very rapid 



