154 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



complete bilateral symmetry and is divisible into head, trunk, 

 and tail, between which there are no sharp lines of demarcation. 

 The head is flattened from above downwards, and ends anteriorly 

 in a rounded snout. The trunk and tail are laterally flattened, 

 while the latter is extremely long and very narrow in its posterior 

 part. 



A number of thin flat fins are present, some unpaired and 

 situated in the median plane, others paired and lateral. They 

 are all supported by an internal skeleton. The unpaired fins are 

 four in number two dorsals, a caudal, and an anal. They are to 

 be looked upon as surviving portions of a continuous expansion 

 which in ancestral forms probably ran along the dorsal surface, 

 round the tail, and forwards for some distance along the ventral 

 surface (cf. Amphioxus, p. 136). The first dorsal is a small trian- 

 gular flap commencing about half-way back along the upper 

 surface ; not far behind it is a similar but smaller second dorsal. 

 The caudal fin fringes the tail and is markedly asymmetrical 

 (heterocercal). It consists of a square-ended upper lobe into 

 which the upwardly bent end of the body is continued, and a 

 rather broader lower lobe. The anal fin projects from the ventral 

 surface opposite the space between the first and second dorsals. 



The paired fins are four in number, and are homologous to the 

 fore and hind limbs of terrestrial Vertebrates. They are probably 

 specialized portions of continuous lateral fins which existed in 

 ancestral forms (cf. Amphioxus, p. 136). The anterior pair, or 

 pectoral fins, project horizontally from the sides of the broadest 

 region of the body, and mark the junction of head and trunk. 

 Each is a broad flat plate with dorsal and ventral surfaces, and 

 (when it is pulled slightly outwards) anterior (pre-axial), posterior 

 (post-axial), and external margins. 



The much smaller pelvic fins are attached to the ventral side of 

 the body, half-way between the snout and beginning of the caudal 

 fin. Their post-axial margins touch each other in the female and 

 are fused together in the male. A ready means of distinguishing 

 the sex is thus afforded, and further, in the male, a part of each 

 pelvic fin is converted into a grooved rod or clasper, which func- 

 tions as a copulatory organ. 



Apertures. The mouth is a large crescentic slit on the under 

 side of the head; the cloacal aperture is an elongated opening 

 between the pelvic fins. There is a small abdominal pore on each, 



