364 MANUAL OF ZOOLOGY. 



branchial arches, instead of the comb-like plates of the typical 

 bony fishes. The endoskeleton is only partially converted into 

 bone, and the exoskeleton, by way of compensation, consists 

 of ganoid plates. The swim-bladder is destitute of an air- 

 duct. 



Fig. 137. Ostraciontidae. Horned Trunk-fish (Ostracion comutus). 



The singular Sea-horses (Hippocampida\ now kept in most of 

 our large aquaria, belong to this sub-order, but the only point 

 about them which requires notice is the curious fact that the 

 males in this family are provided with a sort of marsupial pouch, 

 into which the eggs are placed by the female, and to which the 

 young, when hatched, can retire if threatened by any danger. 

 This singular cavity is only found in the males, and is situated 

 at the base of the tail. More familiar than the Sea-horses are 

 the Pipe-fishes (Syngnathida), of which one species occurs com- 

 monly on our shores. 



CHAPTER LVII. 



GANOIDEI. 



ORDER IV. GANOIDEI. The fourth order of fishes is the 

 large and important one of the Ganoid fishes, represented, it 

 is true, by few living forms, but having an enormous develop- 

 ment in past geological epochs. For this reason the study of 

 the Ganoid fishes is one which claims considerable attention. 



The order Ganoidei may be defined by the following charac- 

 ters : The endoskeleton is only partially ossified, the verte- 

 bral column mostly remaining cartilaginous throughout life, 

 especially amongst the extinct forms of the Palaeozoic period, 

 in which the notochord is persistent. The skull is furnished 



