242 



YERTEBHATE AXUtALS. 



to another order) is the conversion of the bodies of the vertebra' into 

 bone eiinied further than this. 



2. Tlie integument usually develops scales, and these in the great 

 majority of cases are of the forms known as "cycloid " and "ctenoid," 

 the former being circular or elliptical horny j)lales, with jilain mar- 

 gins ; whilst the latter have their hinder margins cut into comb-like 

 projections or fringed with spines (fig. 164, a, h). 



3. The anterior mid postvrinr limbs (lig. 174) are usually, bvit not 

 always, ]iresent, and when develnjied they are alw.iys in the form of 

 fins. Tliese fins may be sujiported by "s]iinous rays " or "soft r.iys," 

 or by both. The s]>iiious rays are simple undivided bony spines 

 which ta|)er to a yjoint. The soft rays are doubly divided, splitting 

 up towards their extremities into a number of secondary r:iys, 

 and being also divided by transverse joints into numerous short 

 jiieees. 



4. liesides the " jiaired " tins wliich represent the limbs, there is 



Fig. 174. -The Cod (^Gadus morrhun). 



.■dso a series of unpaired or "median" fins, the I'ays of which are 

 su]iported upon a series of d.igger-sh.iped bones, deeply plunged in 

 the flesh in the middle line of the Ijody, and known as the " inter- 

 s])inou3 " bones (tig. l()."i). The median fins (fig. 174) are variable in 

 number, but when fully develojied they consist of one to thi-ee fins 

 on the back (the diir.sal fins), one or two on the ventral surface (the 

 anal fins), and one clothing the posterior extremity of the body (the 

 caudal fin, or tail). In all the Tflenxti'i, the caudal fin has the shape 

 called " homoeere.-d " — that is to say, it consists of two ei|Ual lobes 

 — and the vertebral column is not prolongeil into the upjier lobe 

 (tig. 174). 



5. The heart consists of two e.ivilies, an .-luriele and :i ventricle, 

 but the hidhtts arferiiixiix is not rhythmically contractile, and is 

 se|i.uateil from the ventricle by only a single pair of valves. 



li. The res]iiratory org.ans are in the form of free, eomb-like, or 

 tufted gills, enclosed in two cavities placed on the sides of the neck. 



