THE INFANCY OF FISHES 223 



and the housing of its young, at any rate for a brief 

 period, on the common pond mussel ! 



The female, at the breeding season, develops a long 

 tube or ovipositor, which forms a sort of funnel en- 

 abling her to deposit her eggs between the opening of, 

 the shell valves of the mussel. Having reached the 

 interior, these eggs settle down between the folds of the 

 " mantle " or gills of the mussel, and here they remain 

 till they hatch. The young, for about a month, continue 

 to occupy this strange nursery, being closely packed 

 away as shown in our illustration. In due course, how- 

 ever, they escape to complete their growth in normal 

 fashion. The gain to the eggs and larvas from this 

 temporary lodgment is clear, for both protection and a 

 constant supply of fresh water are assured. 



But the mussel exacts a quid pro quo from the bitterling ; 

 for at the time the fish is seeking to secure safe harbourage 

 for her offspring the mussel, having a prodigious number 

 of young of her own just ready to launch forth into the 

 world, and needing temporary assistance, discharges her 

 fry upon the mother-fish. Being armed with a curiously 

 toothed shell, more or fewer of the young mussels succeed 

 in hooking themselves on to the skin of the fish. Herein 

 they become speedily embedded by the swelling of the 

 skin caused by the wound of the tiny shell. Thus 

 ensconced they undergo further transformation, and 

 finally from their increased size burst the walls of their 

 temporary prison and fall to the bottom of the stream, 

 to grow into mussels and found new colonies in due 

 season. 



In the course of these pages a number of really extra- 

 ordinary forms of nursing have been cited, and some 

 striking instances of what we may call infant precocity have 



