its ventral oxtrcniity lies on tlio prooporoiilum. It is so 

 situated that its attached base is exactly opposite to the 

 dorsal jiortion 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 

 slig'htly dorsal. The first branchial cleft is therefore 

 bounded posteriorly by the anterior demibranch of the 1st 

 liranchial arch and anteriorly by the pseiidobraneh. The 

 afferent pseudobranchial vessel or hyoidean artery runs 

 along the external or deep-seated part of the base of the 

 pseudobranch, and gives oif 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. Each is made up of a fiattened axis, 

 on each side of which are borne a number of lamellae. 

 The afferent filamentar vessel runs down the internal 

 (with respect to the attachment of the organ to its arch) 

 edge of this axis ; the eft'erent vessel runs up the external 

 edge. Small twigs are given off from the aff'erent vessel 

 into each lamella, and in each of the latter they break up 

 into a capillary plexus, as in the true gills, Avhich emjjties 

 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 lamellse are mostly 

 free, but many are attached together by their edges. 

 They dift'er from the lamelhe of the true gills in that 

 tlieir wall instead of being a s(|uamous epithelium is made 

 II 



