CHAPTER XXXIX 
SERIES PLECTOGNATHI 
mP|HE Plectognaths.—Derived directly from the Acanthu- 
ride, 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 Plectognatht. As the members of this group differ 
from one another more widely than the highest or most 
generalized forms differ from the Acanthuride, we do not regard 
it as a distinct order. The forms included in it differ from 
the Acanthuride much as the swordfishes differ from ordinary 
mackerel. The Plectognatht (xzAexros, woven together; yvaéos, 
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 Squamtpinnes. 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 vertebrae 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. 
The three suborders of plectognathous are easily recognized 
by external characters. In the Sclerodermi (oKdepos, hard; 
dépua, Skin) the spinous dorsal is present and the body is 
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