CHAPTER XXIV 

 SERIES PLECTOGNATHI 



HE Plectognaths. Derived directly from the Acanthu- 

 rida, from which they differ by progressive steps of 

 degeneration, are the three suborders of Sclerodermi, 

 Ostracodermi, and Gymnodontes, forming together the series or 

 suborder of Plectognathi. As the members of this group differ 

 from one another more widely than the highest or most 

 generalized forms differ from the Acantkuridcs, we do not regard 

 it as a distinct order. The forms included in it differ from 

 the AcanthuridcB much as the swordfishes differ from ordinary 

 mackerel. The Plectognathi (nXe/tro?, woven together; yvdftos, 

 jaw) agree in the union of the maxillary and premaxillary, 

 in the union of the post-temporal with the skull, in the great 

 reduction of the gill-opening, and in the elongation of the pel- 

 vic bones. All these characters in less degree are shown in 

 the Squamipinnes . We have also the reduction and final entire 

 loss of ventral fins, the reduction and loss of the spinous dor- 

 sal, the compression and final partial or total fusion of the 

 teeth of the upper jaw, the specialization of the scales, which 

 change from bony scutes into a solid coat of mail on the one 

 hand, and on the other are reduced to thorns or prickles and 

 are finally altogether lost. The number of vertebras is also 

 progressively reduced until in the extreme forms the caudal 

 fin seems attached to the head, the body being apparently 

 wanting. Throughout the group poisonous alkaloids are 

 developed in the flesh. These may produce the violent disease 

 known as ciguatera, directly attacking the nervous system. 

 See p. 182, vol. I. 



The three suborders of plectognathous are easily recognized 

 by external characters. In the Sclerodermi (a-xXepos, hard; 

 skin) the spinous dorsal is present and the body is 



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