i6 



NERVOUS SYSTEM AND SENSE-ORGANS 



e 



/ 



plified. Some of the simpler forms possess a nervous system very 

 much like that of a short Centipede, and from this condition all 

 degrees of fusion and concentration are found, the maximum being 

 reached where all the ganglia of the ventral cord have united into 

 a single nerve-mass, precisely as in Crabs and Spiders. Three 

 such stages, as exemplified by a Termite, a Water- Beetle, and a 

 Fly, are represented in fig. 1019. In those insects which begin life 

 as larvae, it commonly happens that in tfiis early stage of life the 

 nervous system is simpler than in the adult, exhibiting less fusion 

 and concentration. This is exemplified by comparison of a cater- 

 pillar with the butterfly or moth 

 which it becomes, or a bee-grub 

 with an adult bee. Cases are 

 known, however, where the ner- 

 vous system is condensed both 

 in larva and adult, e.g. the 

 House- Fly and its allies (Miis- 

 cid&). A curious reversal of 

 the ordinary state of things is 

 found in the Ant- Lion (Myr- 

 meleo), for here the nervous 

 system of the relatively short 

 and squat larva is more con- 

 centrated than that of the elon- 

 gated adult. That this should be so is probably not merely due 

 to difference in shape, for the complex habits of the rapacious 

 larva involve elaborate adjustments to the surroundings, which 

 need an efficient and centralized nervous system for their proper 

 performance (see vol. ii, p. 1 1 1 ). So far as we know, the life of 

 the adult is relatively simple. 



It remains to be added that all the air-breathing Arthropods 

 possess a visceral nervous system, which may attain considerable 

 complexity, and takes origin from the nerve-ring. 



NERVOUS SYSTEMS OF MOLLUSCS (MOLLUSCA). The least con- 

 centrated type of nervous system is found, as might be expected, 

 among some of the Primitive Molluscs (Amphineura). The 

 central nervous system of a Mail-Shell (Chiton), for instance, 

 consists of a nerve-ring from which four thick nerves run back 

 (fig. 1020). Two of these are pedal cords, that traverse the 

 substance of the muscular foot, while the others are lateral cords 



A B , c 



Fig. 1019. ; Central Nervous Systems of a Termite 

 (Termes, A), a Water- Beetle (Dytiscus, B), and a Blow- 

 Fly (Musca, c), to illustrate stages in concentration, b, 

 brain; e, eye; v, ventral ganglion. 



