306 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



b. Introduce a small tube into the space between two 

 of these partitions, and force air or water into the cavity. 

 Notice that this fills a narrow space, which runs from the 

 dorsal to the ventral margin, where it ends blindly. The 

 air does not escape laterally, thus proving that the inter- 

 lamellar partitions reach from top to bottom of the gill, 

 and divide its cavity into a number of parallel vertical 

 chambers, the ivater tubes, which are closed below, open 

 above, and separated from each other. 



c. On the side or face of the gill notice the fine parallel 

 lines, which run from the dorsal to the ventral edge. 

 These are the gill tentacles. 



d. Notice also a second set of vertical lines, much far- 

 ther apart than the finer lines ; these indicate the edges of 

 the inter-lamellar partitions. 



e. Cut out a small piece of the gill ; place it on a glass 

 slide ; cover it with water, and with a pair of fine forceps 

 tear away the lamella which is uppermost, and thus expose 

 the inner surface. Wash the portion which remains upon 

 the slide, and then stretch it thoroughly with needles, and 

 examine it with a low magnifying power (fifty to one hun- 

 dred diameters). 



1. In a surface view notice the parallel, brown, torn 

 edges of the inter-lamellar partitions, and between them 

 the more transparent spaces of the water tubes. 



2. Select a part of the specimen where the partitions 

 are somewhat widely separated, and focus a little deeper, 

 thus bringing the inner surface of the wall of the water 

 tube into view. Notice the irregular, scattered, somewhat 

 oval openings, the inner ends of the inhalent ostia, through 

 which the water gains access to the cavity of the water 

 tube. 



3. Focus still deeper, so as to bring the external surface 



