CRUSTACEA. 



131 



Fig. 119. — Two species of Crustacea {Siphonos- 

 toma) which occur as parasites on fishes. The 

 young pass through a nauplius stage much 

 like that shown in Figure 118. 



or with the head deeply sunk beneath the skin. The poor fish cannot 

 escape from them; turn and twist as he may. they adhere as tight as his 

 own scales. They are found on fresh-water fish as well 

 as on salt ; in the Arctic seas as well as in the tropics. 

 Some are small and inconspicuous, while others reach 

 a length of several inches. How much pain and 

 inconvenience they cause we have no 

 means of knowing. It may be that 

 their entrance and growth are so srad- 

 ual that the fish is not seriously incom- 

 moded by them, but still it is probable 

 that had he his own way he would not 

 tolerate their presence. 



Turning now from Cyclops in an- 

 other direction we meet another series 

 of forms microscopic in size and en- 

 closed in bivalve shells. They progress 

 through the water with a skipping mo- 

 tion, and at times thev occur in such 

 immense numbers as to fairly color 

 the water in which thev are found. 

 From the popular point of view they are of but little importance, but still 

 they play their part in the economy of nature. Some of them feed on 

 animals smaller than themselves, while others swallow anything that 

 comes handy — decaying animal matter, the ooze from the bottoms of 

 ponds, or the silt at the bottom of the sea. On this nourishment they 

 wax and grow fat, and then perchance fall victims to the fishes. Indeed, 

 many of our valuable food fishes obtain their whole food supply from these 

 apparently insignificant forms. 



The barnacles are still more distant relatives of the Phyllopods. and 

 in treating of them we are following out one crustacean branch to its 

 farthest limits. They are larger forms than the forms just mentioned, 

 and hence there is more known concerning them and more to say about 

 the in. All of them are marine, and almost all of them are enclosed in a 

 thick, hard, calcareous shell. This latter fact caused them to be regarded 

 by the older naturalists as molluscs. What then was the surprise of the 

 scientific world when, in 1830, Dr. John V. Thompson of Cork announced 

 that these animals were Crustacea rather than molluscs. He for the first 

 time showed the wonderful development through which they pass; the 

 nauplius stage with which they leave the egg and the subsequent free- 

 swimming life which ensues before they become attached. Twenty years 



