310 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



5. Notice that the layer of epithelium which lines the 

 water tube (p) may he traced outwards at (p') until it 

 becomes continuous with that which covers the exposed 

 edges of the tentacles. 



6. The epithelium of the tentacles is greatly thickened, 

 and is made up of a single layer of large cells, which carry 

 the cilia (//, h) which project over the channels between 

 the tentacles. 



7. Behind this thickened epithelium is the somewhat 

 triangular cavity of the tentacle (&). 



8. On the sides of this cavity are the cross sections of 

 the chitinous rods (I). 



9. Back of these rods is the narrow neck connecting the 

 tentacle with the body of the lamella. 



10. The cavity of this neck is traversed in different 

 directions by scattered irregular connective tissue fibres, 

 not shown in the diagram, between which blood-corpuscles 

 will occasionally be found. 



11. The space (r) is occupied by a network of branched 

 connective tissue, through which the blood finds its 

 way. 



d. Make a drawing of the section, showing all these 

 points. 



(iv.) A comparison of the gills of the Cyclas embryo, 

 of Mytilus, and of- Unio, shows that in all of them the 

 gills are made up of a series of parallel tentacles, bent 

 upon themselves to form the two lamellae, and that the 

 inter-lamellar and inter-tentacular junctions, which are 

 slight in Mytilus, are in Unio so much developed as to 

 bind the tentacles into a continuous organ. 



The gill partitions of Unio are thus seen to be homol- 

 ogous with the inter-lamellar junctions of the two halves 

 of a tentacle of Mytilus. The adjacent tentacles of Unio, 



