212 GENERAL SUMMARY. 



procephalic, or first post-oral segment. They become involved during the em- 

 bryonic stages in the palial fold that grows over the forebrain, giving rise, in the 

 vertebrates, to the lateral eye retinas. 



The auditory organs arose from a large midbrain placode near the base of 

 the fourth pair of thoracic appendages. (Limulus.) 



14. The parietal eye is the most ancient of all cerebral sense organs and 

 attains its characteristic structure and location at a very early period in the history 

 of the arachnid phylum. It is usually functional before the lateral eyes have 

 made their appearance. 



.The site of the locomotor organs follows the center of gravity backward, 

 those in the oral region giving place to those of the mesocepTialon ; the latter to 

 those in the postcephalic and caudal regions. The- excretory, digestive, and 

 reproductive organs follow in the same direction. (Fig. 308.) Their nerve 

 centers are of small size and their change of location does not visibly modify the 

 character of the nerve cords. 



The lateral eyes are the next oldest. They are highly developed and impor- 

 tant functionally in the adult stages of nearly all the higher arthropods. During 

 the ostracoderm stage they were involved in the cerebral vesicle, and for a time 

 became practically useless, the parietal eye being the only one that was in a posi- 

 tion to serve as a visual organ. In the true vertebrates the lateral eyes regained 

 their function, and that of the parietal eye gradually disappeared. 



The olfactory organ, while derived from organs probably as old as the parietal 

 eye, did not take on the morphological characters or the function of an olfactory 

 organ till the highest stages in the evolution of the marine arachnids were 

 reached. 



The auditory organ is the most recently acquired. It is dormant in the 

 arachnids, and apparently begins its period of growth and functional activity 

 in the lower vertebrates. It is the only one of the primary sense organs that shows 

 a notable increase in morphological complexity and in the range of its functional 

 activities during the evolution of the vertebrates. 



15. The special cutaneous organs of arthropods include two distinct kinds, 

 the taste organs and the slime buds. Both sets of organs perform various func- 

 tions and initiate various reactions, but all of them may be properly called taste 

 organs, since they react to a variety of chemical substances, either in the food 

 or in the surrounding media. Both kinds may be widely distributed in various 

 parts of the body, but those belonging to this category are sharply localized, and 

 are included, either separately or combined, in segmentally arranged fields that 

 are provided with special nerves and special tracts and centers in the brain. 



In Limulus, taste organs are found in the mandibles of the thoracic append- 

 ages, except the first and last pair, in the chilaria, and the largest field of all in the 

 flabellum. In the scorpion they are found in the maxillaria, the genital papillae, 

 and the pectines. In the mandibles of Limulus, the taste organs and slime buds 

 are located in separate fields, and both sets on stimulation produce chewing reac- 



