4tii'i 



CHORDATA 



lost, though a fusion of digits is not unknown. ralcontologA'. for example, 

 teaches that tlie oiie-loeil liorse has ilescendeil, l>\' gradual reduelion, 

 from live-toed ancestors. 



The completeness and character of tlie skeleton thus outlined has a 

 great inlluenee on the resl of the organism. As alread^• jiointed out, the 

 external appearance of xertehrales lias heen altered \n- it, since the skin 

 is no longer a supporting structure and has consequently lost its segmen- 

 tation. !More immediate is its inlluenee upon the arrangement of the 



Fig. 523. Vjg. 524. 



Fig. 523. — Horizontal section through the anterior trunk region of a young RJimlnis 

 amanis at the level of the Acnlral arclies. c, uolochonl: h, skin; //, ni\osc]ita: ;h, 

 longitudinal nuiscles; r, rib end of the cartilaginous \'entral arch; ■:■, osseous centrum. 



Fig. 524. — Horizontal section of an t.nihr\o of Triton (from O. Hcrtwig). r/;, 

 notochord,- uh, myoccele; i(i,primar)' mu.sclc segment (myotome). 



musculature. The development of an internal skeleton renders it neces- 

 sar}' that the point of resistance of the muscles be transferred from the 

 skin to the interior. A dermal musculature occurs only as an incon- 

 spicuous remnant; it is replaced liy a body musculature. Idris latter 

 consists primarily of a system of longitudinal muscle libres on either side 

 of the verteliral column (fig. 52,;^), which are tlivided by connective-tissue 

 partitions, the iiiyoscfyla, into successixe segments, the ))ivolo))ics. Thus 

 when the eonnecti\e tissue of a lish is dissolved by cooking, the muscles 

 fall into disk-like jiarts. The myosepta extend from skin to axial skeleton. 

 Since they rim ol)li(|uel)' l)ackwards from the skeleton to the skin, they 

 ser\e to render the skeleton a point of resistance for the action of the 

 muscles. 



A segmented trunk musculature occurs in the IShxinoids (and in 

 Ampliio.xus), in which the axial skeleton consists only of notochord and is 

 consequently unjointed. The segmentation of the muscles is therefore ohier 

 than that of the skeleton and, as we can further say, is the cause of it. 



