ARTHROPODA 



217 



SEA-SPIDERS (PYCNOGONiDA ; pantopoda). marine species related to the 

 Arachnids, inhabit the shores or the depths of the seas, Hving on seaweed, 

 hydroids and sponges. They are provided with 4 primitive oceUi perched in 

 two pairs on an ocuHferovis tubercle on the cephalothorax ; as we have already 

 noted, the retinse are of a jieculiar and characteristic inverted type ^ (Morgan, 

 1891 ; Korschelt and Heider, 1893 ; Sokolow. 1911 ; Schlottke. 1933) (Fig. 125). 



Fig. 222. — The Tick, Amblfomma pompo.^vm. 



The two simple eyes, S, lie well posteriorly on the shoulder of the animal 

 (specimen from Xatural History Museum). 



Pj'cnogonid 



IXSECTA 



INSECTS form the largest class of Arthropods and their multitude 

 of types is subdivided with reference to their possession of wings ; it is 

 interesting that the complexity of their eyes varies directly with this 

 characteristic, an association only natural in view of the demands made 

 upon vision by a high degree of mobility. 



(1) Sub-class APTERYGOTA (d, privative ; -nrepv^ a wing), wingle.gs forms, 

 in which through a series of moults the adult differs little from the newly hatched 

 insect except in size. They are the most primitive of insects, some species being 

 marine, and when eyes are present they are simple in type. 



THYSAXURA — bristle-tails, of wide distribution in damp soil, son"ie living 

 between tide-marks or under stones or bark ; others (silver-fish) in 

 bread-bins or books. Closely related are the eye-less diplura. 



PROTURA — ininute creatures (2 mm.) living in moist soils vmder stones 

 and bark, without wings, antennae or eyes of anj^ kind. 



COLLEMBOLA — .sjiringtails, living under stones and leaves ; one species 

 lives between tide-marks. 



(2) Sub-class pterygota, provided with wings which, however, may be 

 secondarily lost through highly evolved specialization. The sub-class is divided 



1 p. 150. 



