66 CLASS PISCES. 



exterior. The gills are either lamelliform (Elasmobranchii) and 

 attached along their whole length to the interbranchial septa, 

 or filiform and projecting (so-called pectinate gills of Teleostei, 

 etc.). They are arranged in a row on each side of the branchial 

 arch, so that each branchial arch carries two rows of gills (holo- 

 branch), one on its anterior and one on its posterior face. 

 Sometimes there is only one row (hemibranch), and some- 

 times gills are absent on each side, or present only as a vestigial 

 structure called a pseudobranch. 



The general arrangement is as follows. The branchial passage 

 between the mandibular and hyoid arches is called the spiracle. 

 It is always reduced and is present only in most Elasmobranchs 

 and some Ganoids. Behind this there follow typically five 



branchial passages or gill- 

 clefts. The mandibular arch 

 never carries more than a 

 vestige of a gill, which is called 

 the mandibular pseudobranch 



v -^v \SNHJBHOHB I \ or pseudobranch of the spir- 

 acle. The hyoid arch never 

 carries more than a demibranch 

 and that on its posterior face. 

 The first four branchial arches 

 carry typically holobranchs, 

 while the last branchial arch 

 is always without a gill. It 

 thus happens that, if the hyoid arch carries a hemibranch 

 on its hinder surface, the first four branchial passages have 

 gills on both anterior and posterior walls, while the fifth 

 branchial passage has a gill only on its anterior wall, the fifth 

 branchial arch being always gill-less. In most fishes, however, 

 the hyoid demibranch is reduced to a vestige, and is then known 

 as the hyoidean or opercular (because the hyoid arch carries 

 the operculum) pseudobranch. Externally projecting gills are 

 found in the embryos of Elasmobranchs and a few Teleostei. They 

 are not true external gills, but are much elongated internal gills. 

 In the young Polypterus and some Dipnoi there appear to be 

 true external gills. 



The brain of fishes is small and does not fill the cranial 

 cavity. It presents all the parts of the typical vertebrate brain. 



