2 86 TEXT-BOOK OF ZOOLOGY 



converting it into a regular broth. (Thsee animals also occur in fresh 

 water in extraordinary quantities, and may be easily caught by means of 



a gauze net.) 



(a) These creatures are so minute (it requires 60,000 to fill a 

 herring's stomach) that we cannot conceive a herring catching and 

 swallowing them singly one by one. They are, in fact, taken in large 

 quantities by the aid of the "gill-rakers." The bony rods of these 

 organs are set very close, and are of considerable length, and being, 



Au. 



HEAD or THE HERRING, OPENED TO SHOW THE OUTERMOST OF THE RIGHT GILLS. 



(Somewhat reduced.) To THE RIGHT, PART or A GILL. (Slightly enlarged.) 

 Kbg., gill-arch; B., branchial lamellae; R., gill-rakers; Rst., three gill-rakers with 

 denticles ; Au., eye-socket. 



moreover, armed with teeth, they form a very fine net, allowing the 

 water of respiration to pass to the gills, but retaining the small 

 crustaceans taken in with the water (compare with the baleen plates 

 of the whale). Thus the herring both breathes and takes in food 

 simultaneously. 



(b) Where these crustaceans are most abundant, the herring, too, is 

 naturally met with in greatest quantity ; and this, again, occurs in places 

 where there is a rich supply of the food of the small crustaceans. The 

 latter live principally on rod-shaped algse or diatoms, which are also 

 found in large quantities in fresh water. (This may be seen by placing 

 under the microscope some of the brown slimy scum which forms in the 

 spring on ponds and pools.) They are very minute plants of highly 

 ornamental structure, of which it requires several millions to fill the 

 space of a cubic centimetre. They are developed specially at the season 

 of the melting of snow in the Arctic seas, and are then driven south- 

 wards by ocean currents. Where these currents are stemmed on 

 submarine banks, or where a cold and a warm current meet these 

 algae are accordingly found in enormous quantities ; and here, too, we 

 meet with the myriads of crustaceans, which in their turn supply the 

 food of the herring. The latter travels to the feeding-grounds in great 

 shoals (food migrations). To accomplish distant journeys like this 

 requires considerable rapidity of movement, of which the herring gives. 



