SEA-FISHERIES LABORATORY. 381 
their ends. Some of the ordinary filaments become 
attenuated and do not reach 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. 25 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, Hel. 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- 
BB 
