tians are, as far as now known, modified diphycercal; that 

 is, with the caudal fin divided into dorsal, ventral, and 

 terminal lobes. It is likely that some earlier actinistians (than 

 the Upper Devonian Diplocercides) had a protoheterocercal 

 tail, that is, one with a reduced but evident epichordal fold. 

 The primitive tail form is assumed to have been proto- 



Figure 6-43. Details of structure of the homocercal toil of a salmon 

 (Blue-backed Salmon 



dorsal fin fold of fulcral spines 



cereal, the caudal fin continuous with the dorsal and anal 

 fins. A protoheterocercal type then developed in which the 

 dorsal and anal fins were separate, and from his came the 

 heterocercal and diphycercal types. The embryological 

 development of the caudal fin suggests this same phylogeny. 

 Most of the early fishes were heterocercal. From this, by 

 reduction of the upturned caudal fleshy lobe, came the 

 homocercal type as well as some of the isocercal types and 

 other modifications. The homocercal type in its turn has 

 been converted to isocercal, abbreviated homocercal, or 

 gephyrocercal. 



These same tail types are observed outside the osteich- 

 thian array. Among the chondrichthians, the tail is usually 

 heterocercal, occasionally isocercal, as in Chimaera or Chlamy- 

 doselachus. The fins of the sharks, and shark-like fishes, ap- 

 pear to be comparable to those of the osteichthian fishes, 

 differing mainly in the type of supporting rays (see Chapter 8). 

 The living agnaths are protocercal in appearance but with- 

 out ray-supported fins. It has been suggested that the early 

 larvae of the lamprey have a slightly hypocercal or down 

 turned tip to the tail (hypos^undftv, beneath, less than). 

 The continuous posterior dorsal and caudal fin of Petromy- 

 zon has been likened to the caudal fin of the anaspid 

 straightened out. Among fossil agnaths the osteostracans 

 were heterocercal, while the anaspids were hypocercal. The 

 heterostracans were originally protocercal, while more ad- 

 vanced types were hypocercal. 



This wide variation of form seen in agnaths has been re- 

 lated to habit. The heterocercal tail tends to elevate the 

 head, while the hypocercal form does just the opposite. 

 Feeding off the bottom is aided by a hypocercal tail, while 

 mid-water swimming of a heavily armored type is aided by 

 the heterocercal type. 



terminal lobule 



1 



posterior ventral lobe 



neural arci 



notochord 

 interventra 



haemal arch 



•.. anterior ventral lobe 



igure 6-44. Details of structure of the heterocercal toil of a sturgeon (Acpenser Mvescens). 



Figure 



170 . THE VERTEBRATE BODY SKELETON 



