97 
its ventral extremity lies on the preoperculum. It is so 
situated that its attached base is exactly opposite to the 
dorsal portion of the first branchial arch, and ihe direction 
of its filaments is almost exactly that of those of the dorsal 
portion of the first holobranch, that is, posterior and 
slightly dorsal. The first branchial cleft is therefore 
bounded posteriorly by the anterior demibranch of the Ist 
branchial arch and anteriorly by the pseudobranch. The 
afferent pseudobranchial vessel or hyoidean artery runs 
along the external or deep-seated part of the base of the 
pseudobranch, and gives off a vessel to each filament. 
The efferent pseudobranchial vessel or ophthalmic artery 
runs along the internal or visible part of the base, and 
receives a vessel from each filament. 
The filaments of which the pseudobranch is composed 
are strikingly similar in appearance to those of any one of 
the true demibranchs, and their structure is the same in 
all essential points. Hach is made up of a flattened axis, 
on each side of which are borne a number of lamelle. 
The afferent filamentar vessel runs down the internal 
(with respect to the attachment of the organ to its arch) 
edge of this axis; the efferent vessel runs up the external 
edge. Small twigs are given off from the afferent vessel 
into each lamella, and in each of the latter they break up 
into a capillary plexus, as in the true gills, which empties 
its blood into a corresponding twig opening into the 
efferent filamentar vessel. 
Only about one-half of each filament projects freely 
into the opercular cavity. The basal halves are all 
attached to each other and to the epithelium clothing the 
inner surface of the operculum. The lamelle are mostly 
free, but many are attached together by their edges. 
They differ from the lamelle of the true gills in that 
their wall instead of being a squamous epithelium is made 
K 
