28 HIGH SCHOOL ZOOLOGY. 



spine, in contradistinction to the other six rays which ai'e 

 jointed and fringed and are therefore known as soft or branched 

 rays. The spine alone is jointed directly to the shonlder-girdle, 

 while the other rays are fixed to it by intermediate pieces of 

 which the hindmost (metapterygial basale) is the largest ; the 

 S[nne in fiict is a ray, plus the foremost intermediate piece, 

 (mesopterygial basale) and their union is to be explained by the 

 use of this ray as a weapon, which may be firmly set (by means 

 of a peculiar joint) and used for offence or defence. 



29. The structui'e of the ventral fin is similar to that of the 

 pectoral. It has eight rays, of which one is hard. There is no 

 pelvic arch, what is generally termed so, being the metaptery- 

 gial basalia of both fins, united in the middle line. 



30. Considerable resemblance to the above will be seen in 

 the fin-rays of the unpaired fins. They are for the most part 

 soft, but some are hard, and they* articulate with the interspi- 

 nals, which again fit into the cleft neural and haniial spines 

 of certain of the vertebrae (§ 13). Of these rays the defensive 

 spinn of the dorsal fin deserves mention, as, from tlie peculia)- 

 arrangement of the interspinals with which it is connected, it 

 may be " set " like the pectoral spine. 



For the purpose of distinguishing different species of fish it is often 

 desirable to count the number of r-ays and express them in a formula, 

 (Roman numerals being employed for the hard, Arabic for the soft rays), 

 e. g. for this species : — 



f D. I, 6 A. 22 



IP. I, 6 V. I, 7 



31. Muscular System. — The muscles of an animal are 

 what we ordinarily call its flesh ; their function is to contract 

 on a stimulus received from the nervous system, and tlius to 

 bring nearer together the parts of the skeleton to which they 

 are attached, or to narrow the tubes round which they are dis- 

 posed. Those surrounding the blood-vessels and intestinal 

 canal form the bulk of what is called the involuntary muscu- 



