304 



ZOOLOGY OF THE INVERTEBRATA. 



Explanatio7i of Fig. 'jg (page joj). 



1. Radial ossicle of the calcareous 

 ring, into which the longitudinal 

 muscle is inserted. 



2. Interradial ossicle of the cal- 

 careous ring. 



3. Radial water-vascular vessels. 



4. Circumoral ambulacral ring. 



5. Polian vesicle. 



6. Two stone canals ending in madre- 

 poric plates ; the upper one is 

 attached to the dorsal mesentery, 

 the lower one hangs freely. 



7. Circumoral blood-vessel. 



8. Ventral blood-vessel. 



9. Dorsal blood-vessel. 



10. Anastomosing branch between 

 different parts of the ventral 

 blood-vessel. 



11. Anterior part of alimentary canal. 



12. 12' 12" The three limbs of alimen- 

 tary canal. 



13. Cloaca. 



14. Cloacal opening with five teeth. 



15. Radiating muscles of cloaca. 



16. Organs of Cuvier. 

 17' 17" Respriatory trees. 



18. Dorsal mesentery, with free pos- 

 terior margin. 



19. Generative organs. 



20. Opening of generative duct. 



21. Circular muscles in body- wall. 



22. Right dorsal muscle. 



23. Right ventral muscle. 



24. Medium ventral muscle. 



25. Left ventral muscle. 



26. Left dorsal muscle. 



The tentacular ampullae are omitted ; the mouth is in the centre of the 



divided tentacles. 



Chapter XVI. treats of the Arthropoda, which are divided 

 into two large groups, according to the nature of their breathing 

 organs : — The BRANCHf ata, which breathe by gills and are typically 

 aquatic ; and the Tracheata, which breathe by tracheae or lung 

 books, and are typically terrestrial. 



The Branchiata include but one class, the Crustacea, and 

 as a last specimen of these beautiful illustrations we have selected 

 Fig. 80, Gammarus tieglecius, an animal very nearly allied to the 

 Fresh-water Shrimp, which is well-known to our readers. It 

 belongs to the sub-order Amphipoda. These crustaceans are 

 generally small, but some few of them living in Arctic Seas, or at 

 great depths in the ocean, attain several inches in length. They 

 inhabit both salt and fresh water, and progress by swimming or 

 jumping. The males may usually be distinguished by the develop- 

 ment of their olfactory hairs on the first antennae, by the absence 

 of oostegites, and by the presence of strong prehensile hooks on 

 the anterior thoracic feet. 



The student of Zoology cannot fail to derive a large amount of 

 valuable information from a careful study of this volume. It 

 contains 263 very fine engravings. Those which we have selected 

 are by no means the best ; but they are such as we thought would 

 prove most interesting to our readers generally. 



