PYCNOGONIDAE 461 



Nymphon Fabricius. Body slender, mandibles 3-jointed; pedipalps 

 5-jointed; ovigerous legs present in both sexes and 11-jointed; legs 

 slender, with accessory claws: 45 species. 



N. stromi Kroyer (Fig, 721). Length 15 mm.; spread of legs 14 

 cm. ; ovigerous legs 19 mm. long, stout, slightly hairy : on muddy bottoms. 



N. longitarse Kroyer. Length 7 ^^ 



mm.; spread of legs 65 mm.; entire ^^^^^^^^^\^ ^ Ni ^^^^^^ 



animal extremely slender, ovigerous J y. : / ^"^ \ 



legs especially so: common from Cape • ''•• | ,5" 



Cod to Nova Scotia, in from 16 to 115 ^j^-^^^*^ ^^^^-•'l^^^^^'^^^^^ 

 fathoms. ^^"^^^^^^"-^^^^^^ ^'^^^^^^^^ 



■ N. grossipes (Fabricius). Length V^ f]f\\ y^ 



10.5 mm. ; spread of legs 9 cm. ; color ^V__ // \\i V— -:==:'^ 

 light yellow, often with purple-banded € \\1 ^ 



legs; eyes black, on a prominent tu- ^^:^ U nn ^^ 



bercle; ovigerous legs very slender: ^^sa^ \^ 



very common from Long Island Sound son";'^- il'irnlffi'f 2? p1Sip%i^^^^ 

 to Nova Scotia, in from 12 to 110 fe|f raWtmen!^ ^'^' ^' ^^"^^^^^^^ 

 fathoms. 



Class 3. TRACHEATA. 



Air-breathing arthropods with a distinct head and a single pair 

 of antennae, which respire through tracheae. 



External Characters. — The more primitive tracheates are worm-like 

 animals in which the body is made up of a large number of similar 

 segments, without any division into thorax and abdomen; the head, how- 

 ever, is always distinctly set off. In the insects, the highest members of 

 the group, the body is divided into the three regions, head, thorax, and 

 abdomen. The paired appendages in the primitive tracheates occur along 

 the entire length of the body, a pair extending from each somite, except 

 usually the last one. In the insects, however, as in the Araclmoidea, the 

 abdomen is usually not provided with locomotoiy appendages. The head 

 appendages are a pair of antennae, which are preoral in position, a pair 

 of mandibles, which have no palps, and in most cases two pairs of maxillae. 

 A diagram showing the homologies of the appendages of tracheates and 

 the other arthropods is given on page 323. 



The Tracheata are preeminently terrestrial and aerial animals and 

 breathe atmospheric air by means of tracheae, which open to the outside 

 through pores called spiracles. A very few insects live on the surface of 

 the sea and a considerable number of species are found in fresh water; 

 in the case of the latter, certain insect larvae breathe water, but most 

 aquatic tracheates still breathe the atmospheric air. The class contains 

 3 divisions. 



