§ I SEXUAL CYCLE OF RHODEUS AMARUS ? 123 



hours. The ova are deposited in the mussel one at a time, 

 at intervals varying from one minute to one hour or even 

 longer. There finally follows the reduction phase, which 

 may last between 12 and 24 hours. The entire ovulation 

 phase generally lasts about 48 hours. 



We should here insert a description of the mechanism of 

 erection of the ovipositor. The ovipositor is placed in a rigid 

 state into the exhalant siphon of the freshwater mussel. 

 The ovipositor itself is completely flaccid, and cannot be 

 pushed into the mussel in that state. When a mature ovum 

 arrives at the base of the ovipositor it is pushed into the 

 ovipositor by contraction of the muscles of the urinary 

 bladder. The urine cannot run away through the ovipositor, 

 because the ovum acts as a plug. When the ovum is pushed 

 towards the tip of the ovipositor, that part of the ovi- 

 positor behind it is entirely filled with urine, which causes 

 a certain rigidity. In this rigid state the ovipositor is intro- 

 duced into the mussel, and is able to penetrate deeply enough 

 into the mollusc for the safe deposit of the ovum. As soon 

 as the ovum has been forced out of the ovipositor the liquid 

 under pressure flows away and the ovipositor relaxes com- 

 pletely. 



This course of events was observed in ovulations that had 

 failed, and in which the ova had missed the mussel. In 

 normal cases the ovum glides through the ovipositor so 

 quickly that the tube is rigid for only a very short time and 

 it is difficult to follow the process closely, especially as the 

 ovipositor is for the greater part inside the shells of the 

 mussel (vide Fig. 81 and 82). 



An interesting detail is the behaviour of the male in display and during 

 oviposition. In estrus the male is very finely coloured. On the breast and 

 belly the colours become a yellowish orange to a warm red, while the 

 back- and anal fins grow a deep red and are marked with conspicuous 

 edges. Another striking feature is the steel-blue, irridiating side-line. On 

 the nose, on either side of the nostrils, small chalky-white warts appear, 

 the so-called pearl-form organs, consisting of epithelial cells closely 

 crowded together. When the male displays in spawning time it is inclined 

 to place itself near a mussel. The latter is regarded as the centre of the 

 male's territory, and from here fishes venturing inside this domain are 



