MODES OF REPRODUCTION 327 



are hatched, driving away intruders and fighting fiercely with 

 other males if they come too near. 



The marine stickleback, Gastrosteus spinachia, which is 

 common on British shores and is much larger than the fresh- 

 water species, reaching a length of six to eight inches, not only 

 builds a nest but binds the materials of it together with a thread 

 spun from its own body. The nest consists of growing sea- 

 weeds, and the kidneys of the male in the breeding season secrete 

 a gelatinous substance which hardens as it is drawn out into a 

 strong white continuous fibre, and this is wound about and 

 woven into the nest as the fish swims about it during its con- 

 struction. (Plate XXVIII.) This fish, therefore, may be said 

 to spin a cocoon somewhat in the same manner as a silkworm 

 or other caterpillar or a spider ; the silk of the caterpillar or 

 spider however is secreted by special glands while in the fish the 

 secretion comes from the kidneys ; there are differences also in 

 the use made of the secreted fibre, the caterpillar makes a cocoon 

 only to protect itself in the pupa stage, or as an aid to locomo- 

 tion, spiders very generally spin cocoons to contain their eggs, 

 but many of them also spin webs and snares for the capture of 

 prey. The sea stickleback affords the only instance of spinning 

 among the vertebrates of which we have certain knowledge. 

 Among the floating masses of the sargasso weed in the Sargasso 

 Sea, that is to say in the middle of the North Atlantic, occurs 

 frequently a fish-nest which at first sight seems to be constructed 

 in the same way as the nest of the fifteen-spined stickleback. 

 This nest consists of fronds of the weed held together by threads 

 and containing clusters of fish-eggs, but careful examination has 

 shown that the threads in this case are processes or outgrowths 

 of the egg-membranes, two bundles of them arising from each 

 egg at opposite poles of its surface. The threads must there- 

 fore be produced, as in the eggs of the gar-fish, Be/one acus or 

 Rhamphistoma belone (Plate XXVII., B), and some other 

 species, in the ovary during the development of the eggs. The 

 nest has been shown to be formed of a single plant of the 

 sargasso weed, which is allied to the common Fucus or 

 bladder-wrack of our own coasts, and like it has air-bladders in 

 its fronds which cause it to float. All the complicated rami- 

 fications of the plant are drawn together by the threads so 

 that the nest has a spherical form and is about twice as large 



