49 



their ends. Some of the ordinary fihiments become 

 attenuated and do not reacli the level attained by most of 

 them, so that the number of filaments is reduced. 



These spatulate terminations of the ascending portion 

 of the filaments (figs. 20, 21, Br. a.') are only united by 

 stiff cilia. In one or two cases, however, in a whole series 

 of filaments, two adjacent ones may be organically 

 connected, but this is exceptional, and usually there is no 

 connection between the upper ends for the circulation of 

 the blood from one filament to another. Instead of there 

 being a septum across the filaments at this level, there are 

 numerous delicate strands crossing from one side to the 

 other. Above the level of the reflected lamellae, the 

 branchial interlamellar expansion is developed on the 

 descending principal filaments, but they change their 

 shape somewhat and the plications of the lamellae become 

 deeper. 



The main portion of the principal filament is now 

 roughly rectangular, as fig. 23 shows. The chitinous 

 skeleton is much more strongly developed, the two 

 longitudinal bars can still be made out, but the lateral 

 portions have thickened up considerably and have broad 

 transverse connecting bridges. The pale staining chitin 

 is present in the same position as in the sections cut lower 

 down. Cilia are borne by the frontal edge as before, but 

 owing to the increase in the depth of the plicae the 

 adjacent ordinary filaments have their sides parallel to 

 the frontal margin instead of being in the same relative 

 position with their sides parallel to the sides of the 

 principal filament. 



The nerve (fig. 23, Fil. n.) can be seen embedded in 

 the chitinous skeleton of one side, and there is also more 

 connective tissue and muscle fibres in the filament. 



The branchial expansion, which is on the inter- 



D 



