226 THE STICKLEBACK FAMILY. 



It frequently happens that an already completed nest is 

 selected by a second pair, and the efforts of these in the way of 

 building and of the deposition of ova are superposed upon those 

 of the former couple, resulting in a very much larger nest. 

 Naturalists were for some time puzzled by the fact that many 

 nests contained a greater number of eggs than a single female 

 could produce, but the foregoing explains the condition. 



The binding thread is exuded from the anal region of the 

 male and appears to consist of a modified form of mucilage ; it 

 is, when fresh, colourless and tenacious, and exhibits a delicate 

 opalescence which disappears after a few days, leaving a greyish 

 mass. On close examination, the cord appears to consist of 

 several threads or strands, the cord being from '004 to '005 inch 

 in diameter and the strands from '0008 to '0009 inch in diameter. 

 Each strand may again be observed to be composed of an 

 immense number of little fibrils of a delicate structure, running 

 parallel to one another and fco the long axis of the cord. The 

 male appears to select a suitable mass of seaweed and attach to 

 it by mere contact the viscid mass of mucus protruding from 

 the urinary aperture and then by passing and repassing over 

 and under the growing nest he binds all firmly together. The 

 regular cross-arrangement of the threads is well shown in Prof. 

 Prince's figure of the nest (Frontispiece). 



The origin of the mucoid cords is very suggestive, for an 

 examination of the male in the breeding season (Plate VIII, 

 fig. 10) reveals the fact that the kidneys are swollen and the 

 lining cells of the urinary tubules are found to be engaged in 

 the production of the secretion. Whether they act as secretory 

 cells as suggested or whether they themselves undergo a de- 

 generation such as takes place in the case of many plant-cells 

 in the production of mucilage, it would be difficult to say. In 

 either case, the product of an excretory organ has been made 

 dii-ectly available by the organism to subserve a function 

 important to the well-being of the species, a striking example 

 of the strict economy practised in nature. The mucous secre- 

 tion passes from its point of origin down the ureters to the 

 urinary bladder, in which it is stored up in large quantities. 

 Finally it emerges through the urinary aperture, as an elastic 



