XII] 



ARTHROSTRACA 



223 



As in the case of the Decapoda some genera of Isopoda have 

 forsaken the sea for a life on land, amongst which the Wood-lice, 

 Oniscus and Porcellio, exhibit certain peculiarities usually associated 

 with Insects; thus the mandible has no palp, one pair of antennae 

 is usually lost, and there are certain tubular air passages believed 

 to be respiratory burrowed in the abdominal endopodites which 

 recall by their structure the tracheae of air-breathing Arthropoda. 

 This is another proof that any attempt to group together all animals 

 possessing tracheae leads to absurdities. 



24 



22 



123 



,:XIV-XIX 



FIG. 89. Gammarus neglectus. Female bearing eggs seen in profile. 

 Leuckart and Nitsche, after G. 0. Bars. 



From 



i-vi. Cephalothorax. vii-xin. Free thoracic segments. xiv-xix. The 

 six abdominal segments. 1. Anterior antenna. 2. Posterior antenna. 

 3. Mandibles.. 4. 1st maxilla. 5. 2nd maxilla. ' 6. Maxillipede. 

 713. Thoracic limbs. 1416. Three anterior abdominal limbs for 

 swimming. 17 19. Three posterior abdominal limbs for jumping. 



20. Heart with six pairs of ostia. 21. Ovary. 22. Hepatic diver- 

 ticula. 23. Posterior diverticula of the alimentary canal. 24. Median 

 dorsal diverticulum. 25. Alimentary canal. 26. Nervous system. 

 27. Ova in egg pouch, formed from lamellae on the coxae of the second, 

 third and fourth thoracic limbs. 



