MINUTE STEUCTUEE OF GILLS OF PALiEMONETES VAEIANS. 77 



The Axis. — A transverse section through the axis of a gill 

 (fig. 1) has a triangular shape, the sections of the lamellae (/.) 

 appearing attached along the two sides of the triangle. The 

 surface of the axis is covered with a layer of chitin {cht.), 

 which is thinner than that covering the body of the animal. 

 Beneath the chitin lies the ectoderm [ect.), consisting of a 

 layer of cells with large oval nuclei. At the apex of the 

 triangular section the main arterial channel is situated (figs. 

 1 and 3, art.), which runs along the whole length of the axis, 

 and supplies blood to the lamellae. This artery is lined by a 

 layer of epithelium {a. ep.) composed of clear cells with rela- 

 tively small nuclei. The epithelium, however, only com- 

 pletely surrounds the vessel at irregular intervals (fig. 3, a. ep.), 

 it being broken in front, for the most part of its length, to 

 allow the blood to pass into the channels of the lamellae. The 

 artery then appears to have its outer wall formed by the cells 

 of the ectoderm (fig. 1). At the point of attachment to the 

 body an artery, also lined with epithelium, enters the gill 

 and unites with the main artery of the axis (fig. 4). 



In the other two angles of the axis run the main venous 

 channels (figs. 1, 2, and 4, vn.), which unite together and 

 enter the body by a common vessel immediately below the 

 artery. Fig. 3 represents a section through this point, which 

 lies a little below that represented in fig. 4. 



The venous channels are surrounded by the connective- 

 tissue cells {con. t.). The latter are large irregular cells, 

 each containing a small, deeply staining nucleus, an outer 

 layer of protoplasm, and a large, clear vacuole, in which a 

 mass of yellow-coloured concrement is deposited. Around 

 the main venous channels these cells form a more or less 

 continuous layer, which is, however, interrupted at various 

 points. In the other parts of the axis they are arranged 

 somewhat loosely, leaving intercellular spaces, which are in 

 direct communication with the blood-channels, and are filled 

 with blood. Fig. 2, drawn from a section near the end of a 

 gill, in which osmic acid has been reduced by pyroligneous 

 acid, the blood being thereby coloured black, will make this 



