224 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
limited circulation of the blood through the filaments 
themselves. 
There is a very regular segmental or repetitional 
structure in each branchia, which is due to the fluting 
and to the regular occurrence of the afferent and efferent 
vessels. At the base of each furrow is a vessel, the 
external wall of which is formed by the two flattened- 
out filaments already referred to. The rest of the vessel 
is formed from extra-filamentar tissue. Afferent and 
efferent vessels so formed alternate with complete regu- 
larity along the whole length of the branchia. There is 
this difference between the two series :—the afferent series 
(Br.aff’.) consists of a number of hollow plates extending 
uninterruptedly from the base to the ventral edge of each 
branchia, and also stretching across from external to 
internal lamelle; each afferent vessel is thus common to 
the two lamellez of the branchia; the efferent series consists 
of a number of nearly cylindrical vessels (Br.eff’.), one of 
which is present at the base of every alternate furrow. 
They must necessarily be double the number of the 
afferent vessels. The afferent vessels form the inter- 
lamellar junctions. It also follows from this arrangement 
that the suprabranchial cavity in the inter-lamellar space 
is divided up into a series of separate cavities having no 
connection with each other except at the base of the 
branchia. 
Fig. 26 represents such a double segment in the internal 
branchia of one ctenidium. ‘There are from 50 to 60 such 
segments in the length of the organ. 
The separate filament (fig. 27) is in transverse section. 
ellipsoidal in shape, the broader end is external, the more 
pointed end is internal. The outer surface is composed of 
large cubical cells, the inner surface of smaller cells. 
Within is an elongated cavity across which bridges of 
