CHAPTER II. 



OUTLINE OF ARACHNID THEORY; CONTINUED. 



I. Comparison of Adult Arthropods With Adult Vertebrates. 



The preceding analyses have shown, that beneath a heavy disguise of con- 

 tour and surface detail, the structural plan of an arachnid and of a primitive 

 vertebrate is after all the same. Let us now consider several types of adult 

 arthropods and see how they compare with vertebrates. 



I. Orientation of Neural and Haemal Surfaces. 



It will be seen that although the location of the eyes and the shape of the 

 body indicate the usual position of the animal during locomotion, they afford no 

 certain evidence as to which is the neural and which the haemal surface, for the 

 pattern formed by the sense organs on the neural surface of the cephalothorax 

 of some arthropods may be very similar to that on the hasmal surface in others, 

 and this fact must be borne in mind when comparing them either with ostraco- 

 derms or with true vertebrates. 



In the phyllopods, and in many other Crustacea that swim neural side up by 

 means of oar-like cephalic appendages, the center of gravity usually lies below 

 the attachments of the swimming appendages. In such cases the parietal ocelli 

 and lateral eyes lie near their original embryonic position, on the upper, or neural 

 surface, as they do in vertebrates. (Figs. 7, 9, 244, 247 and 260.) 



Where locomotion is effected either side up, as in Limulus, the prevalent mode 

 of life may be indicated by the position of the eyes and legs, and by the shape of 

 the body. Limulus, for example, uses its sixth pair of legs as pushing poles, 

 as it moves over soft bottoms, or crawls along partly buried in sand, with 

 little more than the median and lateral eyes exposed. During the adult stage, 

 however, it frequently swims, neural side up, for considerable periods, and per- 

 sistently does so in the larval or trilobite stages, the sloping, anterior margin of 

 the shield, like a well turned bow of a boat, holding the head up and the body 

 properly balanced. The same modes of life and dual methods of locomotion 

 undoubtedly occurred in many trilobites and merostomes, and when the free 

 swimming life predominates, one or more pairs of appendages are enormously 

 enlarged to form heavy, oar-like swimming appendages. The lateral eyes may 

 then lie well forward on the head, between the neural and the haemal surfaces 

 (Fig. 5)- 



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