ORDER SELACHII; GENERAL CHARACTERS. 31 



spines, scattered more or less abundantly over the surface of the 

 body. The head is never covered with bony plates. 



577. The structure of the bulb of the aorta has already been 

 noticed ( 573). The gills are completely fixed, the branchial 

 arches being immoveable, and the gill-laminae attached by one of 

 their margins to a series of partitions. These form several separate 

 branchial sacs, into each of which the water penetrates from the 

 pharynx through a separate slit, and from which it issues by a 

 corresponding external aperture. Of these orifices, which are 

 situated behind the head, there are usually five on each side ; 

 but some Fishes have six or seven ; whilst in the curious Chi- 

 nMeras, the water used in respiration is discharged through a 

 single aperture, although the structure of the branchiae is the 

 same as that of the typical species of the order. 



578. By a peculiar arrangement of the reproductive organs, 

 the majority of the Fishes of this order produce living young ; but 

 a considerable number are oviparous, although in all cases the 

 eggs are fecundated before their deposition. The eggs of the 

 oviparous species are enveloped in curious oblong, horny shells, 

 which are generally provided with filamentous appendages at 

 the angles ; which are said, by clinging to sea-weeds and other 

 sub-marine objects, to protect the young animal from being driven 

 about and injured by the force of the waves. The empty cases 

 are frequently cast up on the sea-beach ; they are well known 

 under the names of mermaid's purses or sea-purses, and generally 

 regarded by seaside visitors as some curious kind of sea-weed. 



579. Besides the ordinary Sharks and Rays, this order in- 

 cludes a singular small group of fishes, the Chimaeras, of which 

 only a very few species are known. These, however, differ from 

 the ordinary Selachii in so many remarkable characters, that they 

 are regarded as forming a distinct sub-order, leading towards the 

 ordinary fishes. The PLAGIOSTOMI, or typical Selachii, are readily 

 distinguished by their possession of several branchial openings on 

 each side ; whilst in the HOLOCEPHALI, or Chimceras, the water 

 passes off from all the branchiae of each side by a single orifice. 

 The latter form only a single family ; but the Plagiostomi are 

 divided into two primary groups, which are further subdivided 



