54 -^ Guide to the Zoological Collections in the 



which usually preserve their purely locomotor function and 

 structure, but are sometimes modified for prehension, and 

 even for the mastication of food. 



The abdominal region forms the posterior part of the 

 body and consists of numerous segments, which are but 

 little modified in form, and are in some groups {e.g., the 

 Insects and Spiders and some Crustacea), quite devoid of 

 appendages. 



The alimentary canal has very much the same general 

 form that it has in the Annelida^ as has also the nervous 

 system ; and this can be verified by inspection of the letter- 

 ed dissectio ns in Case 54, which is occupied entirely by 

 preparations illustrating the internal morphology of the 

 Arthropoda. The nervous system of the Arthropod is, 

 however, much more highly developed, and sense-organs 

 (feelers or antennae, eyes, olfactory and auditory organs) 

 are usually present. The breathing organs differ greatly 

 in the different groups : in Peripatus, in the Centipedes, 

 and in the Insects, air is taken in through numerous small 

 tubes, known as tracheae, which open to the exterior at 

 various places on the surface and ramify throughout the 

 tissues of the body : in the Spiders air usually passes in 

 and out of pocket-like depressions known as pulmonary 

 sacks : in the Crustacea breathing is effected by means of 

 gills. The different forms of breathing organs — tracheae, 

 pulmonary sacks and gills — are specially shown in several 

 preparations in Cases 54, 60, 62 and 63. 



The blood, which is colourless, commonly circulates in 

 a distinct system of blood-vessels, and is often propelled 

 by a contractile heart which is placed in the dorsal half of 

 the body, as is shown in the dissection of the common Crab, 

 and common freshwater Shrimp in Case 54. 



Reproduction is usually sexual, but many forms of 

 asexual reproduction occur among the Arthropoda, espe- 

 cially in the Class of Insects. 



