VERTEBRATA. 5 7 



SUB-KINGDOM. VERTEBRATA, 



Division I. ANALLANTOIDEA. 



Sub-division I. ICHTHYOPSIDA. 



Class PISCES. 

 Order TELEOSTEI. 



92. A common Perch (Perca fluviatilis). 



The roof of the cranial cavity and the greater part of the 

 left wall of the abdomen have been removed, exposing the 

 brain and most of the internal viscera. The following external 

 features may be noticed : the absence of distinct divisions be- 

 tween cephalic, thoracic and caudal regions; the imbricated 

 scales of the variety called ctenoid; the paired, and median 

 unpaired fins supported by hard, unbranched, non-striated rays, 

 and by soft, branching, striated rays ; the large size of the eyes ; 

 the absence of distinct eyelids; the presence of persistent 

 visceral clefts interposed between the visceral arches, and pro- 

 tected partly by a long operculum and partly by a membrane 

 supported on a series of branch io-stegal rays. Bristles are 

 inserted into the paired caecal nasal sacs, each of which has 

 two apertures. The pectoral fins are situated a little behind 

 the free edge of the operculum ; and behind these again are 

 the paired ventral fins. All the rays of the pectoral fins are 

 soft, but the first ray of the ventral fin is hard. The mesial 

 fins are the dorsal, divided into two divisions, the anterior of 

 which is composed entirely of hard rays, as is the rule in 

 Acanthopterygian fishes, the caudal, and the anal. "A series 

 of scales perforated by mucous ducts supplied with nerves forms 

 a lateral line along the trunk, the contour of which corresponds 

 with that of the upper line of the back." (Rolleston.) A portion 

 of the skin has been removed from the lateral surface in the 

 vicinity of the head thus exposing the lateral branch of the 

 pneumogastric nerve, which passes posteriorly in relation with 

 the lateral line. A slip of blue paper has been placed beneath 

 this nerve. The tendinous intersections dividing the great 

 lateral muscles into a series of segments, may also be noticed. 



