308 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



5. Focusing a little deeper, notice that each tentacle is 

 a tube, with a cavity which is irregularly divided by con- 

 nective tissue fibres, among which white blood-corpuscles 

 may occasionally be found. 



4. In order to gain a clear conception of the relations 

 of the parts of the gill, it is necessary to study sections 

 of hardened specimens. The more important points are 

 readily shown in sections of gills which have been placed, 

 for twelve hours, in a three-tenths of one per cent solution 

 of chromic acid ; and the hardened gills may be preserved 

 in ninety per cent alcohol. 



a. Examine a transverse section, that is, one across the 

 water tubes, with a low power. Note : — 



1. The two lamellae (^B and C, Fig. 154) , bound together 

 at intervals by the inter-lamellar partitions ( J5', E, E) . 



2. The water tubes (A, A, A). 



mmii 



,<5 



ii>^ri)f^"r^/ — raf.-.norsior>,— -"C 



J?. \ ry J}. 



r 



Fiti. 154. 



Fig. 154. — Transverse section of the gill of Unio purpurea, magnified 

 eighty diameters. (Drawn from nature by W. K. Brooks.) 



A, A. Water-tubes. B. Outer lamella. C. Inner lamella. D. Blood- 

 vessels. E. Inter-lamellar partitions. F. Inhalent ostia. g. Gill-ten- 

 tacles. 



3. In some of the partitions, the cut sections of blood- 

 vessels (Z>, D). 



4. The outer surface of each lamella is seen to be folded 

 or corrugated, thus forming a series of rounded promi- 

 nences {G, G, G), the sections of the gill-tentacles. 



5. Between these tentacles are the furrows, which vary 



