186 THE FRESH- WATER MUSSEL [CH. VI 



gill-plate is, under normal conditions, applied to the 

 visceral mass in this region so as to enclose a temporary 

 tube one of whose walls is formed by the above-mentioned 

 belt of specialised cilia. In the course of about 50 seconds 

 an egg is thus swept back to the slit between the protractor 

 muscle of the shell and the point of fusion of the right and 

 left inner gill lamellae ; here they meet the stream of ova 

 from the other side of the body and so reach the exhalant 

 current and the cloaca 1 . 



The process goes on for some 10 days or more in each 

 individual and the number of eggs is immense. The 

 estimates of various authorities range from 14,000 to 

 1,000,000 ; probably half a million may be taken as a fair 

 average. On reaching the cloaca the eggs do not, as might 

 be expected, pass out of the shell with the outflowing 

 stream of water. On the contrary their direction is 

 reversed and they pass forward into the cavities of the 

 right and left outer gill-plates, which serve as brood 

 pouches. The method by which this change of direction 

 is accomplished is not quite clear. I have not been able 

 to detect any reversal of the ordinary ciliary currents, 

 nor can I accept the suggestion of von Baer, that the 

 eggs accumulate in the cloaca and pass forward into 

 the outer gill as the result of pressure due to their 

 own accumulation. This theory necessitates the closure 

 of the shell and consequent cessation of respiration during 



1 "Cloaca" is the term applied to a cavity common to the posterior 

 portion of the digestive system and some other system or systems such 

 as the excretory or genital or both of these. Here it is composed of the 

 terminal parts of all systems which convey matter out of the body. 



