COMMON PERCH. 85 



An Acanthocephalous parasite, Echinorhynchus Proteus, is not uncom- 

 mon in the intestines. E. angustatus is also found in the same place as 

 well as the Nematode Cucullanus elegans and the remarkable Cestode 

 Triaenophorus nodulosus. A variety of other parasitic worms may be met 

 with, a list of which is given in Zschokke's work cited below (p. 90) or in 

 von Linstow, Compendium der Helminthologie, Hannover, 1878, p. 206, an 

 invaluable work for all students in this difficult branch of Zoology. 



The epidermis consists of several layers of cells, which are united by proto- 

 plasmic processes. The external cells generally bear a striated cuticle. Glandular 

 cells are present and open on the surface. There are also ' Retort-cells,' or ' Kolben- 

 zellen,' with clear contents, which rise to the surface and burst. The scales are 

 dermal ossifications, and doubly refractile. They are covered by the epidermis, 

 and contained within pouches of the dermis. When the free border is evenly, or 

 nearly evenly round as in the Perch, they are said to be cycloid ; when it is pro- 

 duced into more or less prominent teeth, they are said to be ctenoid, as in many 

 other Acanthopteri. The scales of the lateral line are modified. They are per- 

 forated for the passage of a nerve, and channelled on the outer surface near the 

 free edge. The channel towards the base of the scale is converted into a canal for 

 the protection of the sense-organs of the lateral line. 



These organs are composed of two sets of elements : (i) short pyriform sense- 

 cells terminating in a sensory hair at their outer free extremity, and a nerve-fibril at 

 their inner ; (2) supporting cells which are long, and reach the cutis, and secrete a 

 limitans externa on their outer surface, which is pierced by the sensory hairs. 

 These elements are grouped into eminences or ridges, which are connected in the 

 lateral line in many instances by either modified epidermis, or by non-medullated 

 nerve fibres. The eminences may occur on every scale of the body, e. g. in the 

 Grey Mullet ; or they may be grouped along the lateral line, while isolated eminences 

 occur here and there on the body, e. g. in the Pike, where they are found in numbers 

 towards the tail. In the Perch they are found only in the lateral line of the body, 

 and in its continuation, the mucous canal system of the head. A single canal 

 runs along the post-temporal scale. It divides, and one branch traverses the upper 

 fork of the scale, and crosses the parietal to the opposite side of the body. A 

 second branch runs along the edge of the pterotic bone, and divides into a supra- 

 orbital branch which pierces the frontal bone, and runs above the nostril along 

 the nasal bone, and an infra-orbital branch which runs along the chain of sub- 

 orbital bones, and forward through the homologue of the lacrymal (?), and as a 

 rule unites with the supra-orbital branch in front of the anterior nostril. The third 

 main branch, the infra-maxillary, runs down the praeoperculum externally, crosses 

 the articular, and perforates the dentary bone. The position of the sense bodies 

 on these branches is not very evident in the Perch. There appears to be none on 

 the connecting branch, two only on the supra-orbital, one just behind the posterior 

 narial opening, and a second between the two apertures ; four may be discovered 

 near the free edge of the lacrymal (?), three or four on the praeoperculum, and 

 four very evident in the dentary region. There are as a rule in the Perch no 

 perforations corresponding to these bodies. They exist however in the Pike, and 



