12 AN INTRODUCTION TO ENTOMOLOGY 



developed in some forms, especially those that are found in very deep 

 water, i. e. below four or five hundred fathoms. When well-developed 

 they are simple, and consist of two pairs, situated on a tubercle, on 

 the head or the first compound segment of the body, the segment that 

 bears the first four pairs of appendages. 



The reproductive organs open in the second segment of the legs. 

 In some these openings occur only in the last pair of legs ; in others, in 

 all of the ambulatory legs. 



Very little is known regarding the habits of these animals. The 

 most interesting features that have been observed is perhaps the fact 

 that the males carry the eggs in a mass, held beneath the body by the 

 third pair of appendages, the ovigerous legs, and also carry the young 

 for a time. 



As to the systematic position of the class Pycnogonida, very little 

 can be said. These animals are doubtless arthropods, and they are 

 commonly placed near the Arachnida. 



CLASS TARDIGRADA 

 The Tardigrades or Bear Animalcules 



The members of this class are very minute segmented animals, with 

 four pairs of legs, but without antennae or mouth-appendages, and without 

 special circulatory or respiratory organs; the reproductive organs open 

 into the intestine. 



The Tardigrada or tardigrades are microscopic animals, measuring 

 from one seventy-fifth to one twenty-fifth of an inch in length. They 

 are somewhat mite-like in appearance; but are very different from 

 mites in structure (Fig. 13 and 14). 



The head bears neither antennae nor mouth-appendages. The 

 four pairs of legs are short, unjointed, and are distributed along the 



entire length of the body, the 

 fourth pair being at the cau- 

 dal end. Each leg is termin- 

 ated by claws, which differ in 

 number and form in different 

 genera. 



The more striking features 



of the internal structure of 

 Fig. 13. A tardigrade (After Doyere). ,, . - . , . 



these animals is the absence of 



special circulatory and respiratory organs; the presence of a pair of 

 chitinous teeth, either in the oral cavity or a short distance back of 



