xm 



PHYLUM CHORDATA 



223 



Trout, with a more or less disguised asymmetry : in many cases 

 in the adult the development of the large, fan-shaped, posterior 

 heemal arches- completely hides the upturned end of the notochord, 

 and in some the spinal column ends simply in a somewhat compressed 

 centrum around which the fin-rays are symmetrically disposed ; 

 such truly symmetrical tail-fins are called dipliycercal. 



In the* structure of the skull the Chondrostei make the nearest 

 approach to Elasmobranchs. The cranium (Fig. 897) is an 

 undivided mass of cartilage with a few isolated replacing bones. 

 The roofing investing bones lie in the dermis, so as to be practi- 

 cally superficial, and behind pass insensibly into the scutes covering 

 the trunk : the fact that these bones (parietals, frontals, &c.) 

 are exoskeletal structures is here perfectly obvious. The same is 



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FIG. 8'J7. Skull of Sturgeon, with the investing bones removed, .a, pharyngo-branchials ; 

 AF, antorbital process ; AR .articular ; b. epibranchial ; c. ceratobranchial ; C, notochord ; 

 Cop. basibranchials ; d, hypobranchial ; De. dentary ; OK, auditory capsule ; HM. hyoman- 

 dibular ; hy. hyoid cornu ; Ih. interhyal ; MA. mandible ; Na. nasal capsule ; 06. neural 

 arches ; PF , post-orbital process ; PQ. palatoquadrate ; Ps. Ps'. Ps". parasphenoid ; Psp. 

 neural spines ; Qu. quadrate ; JR. rostrum ; Ri. ribs ; Sp. N. foramina for spinal nerves ; 

 Sy. symplectic ; WS, vertebral column ; x. vagus foramen ; / V, branchial arches. 

 (From Wiedersheim's Comparative Anatomy.) 



the case in Polypterus (Fig. 898), in, which, however the replacing 

 bones are better developed. In Lepidosteus and Amia, and 

 especially the latter, the skull resembles that of the Trout in all 

 essential respects, the main differences consisting in the absence 

 of certain bones, such as the supra-occipital, and in the presence 

 of additional investing bones. Among Teleostei it is only in the 

 Physostomi that the investing bones remain separable from the 

 chondrocranium in the adult ; in the remaining orders, e.g. in the 

 Cod, Haddock, or Perch, they become grafted on to the chondro- 

 cranium and so closely united with the replacing bones that they 

 can be removed only by pulling the whole skull to pieces ; most 

 of the original cartilage frequently disappears in the adult and 

 the cranium thus becomes a firm bony mass in which no distinction 

 between replacing and investing bones is discernible. 



The varying size of the gape, which is so noticeable a feature in 

 the Teleostomi, depends upon the inclination of the suspensorium : 



