320 THE VOYAGE OF H.M.S. CHALLENGER. 



137 species, are found between 150 and 500 fathoms, whereof seventy-two are confined 

 within those limits, while thirteen descend from below 150 to below 1000 fathoms ; and 

 twenty to below 500. Between 500 and 1000 there have been discovered sixty-four 

 species, whereof one-half, or thirty-two, are confined to those limits, while five descend 

 from the starting-point below 500 to below 1000 fathoms. Finally, sixty-nine species 

 in all get below 1000 fathoms, and of these fifty do not pass above that limit. Of course 

 these numbers are temporary. More dredging will bring more species, and will extend 

 the bathymetric range of many, and increase the proportion of the deep-water species 

 to those of the littoral zone, which hitherto has been much more carefully explored. 

 Nevertheless we may say, in general terms, that a very large proportion live exclusively 

 on the littoral zone, and that therein are included species both of cold and of hot water, 

 though the number of the latter is much the larger. Then there is a large fauna of fifty 

 species, which live exclusively below 1000 fathoms and which have to endure a degree of 

 cold near to freezing, an enormous water pressure, and an entire absence of sunlight. 

 Between these extremes there are large groups whose favourite or even necessary habitat 

 is restricted to given depths. Of the genera mentioned in Table I., Ophioplocus, 

 Ophionema, Ophionephthys, Ophiarachna, Ophiarthrwm, Ophiomastix, Ophiopteris, 

 Ophiogymna, Ophiocnemis, Ophiomaza, Ophiothela, Ophiopsammnim, Ophioblenna, 

 Astrophyton, Euryale, and Trichaster, sixteen in all, do not go lower than 30 fathoms, 

 and they, without exception, inhabit warm seas. This proves that certain groups demand 

 a high temperature and cannot accommodate themselves to a lower one. Should any of 

 them, therefore, be found fossil, it would be reasonable to infer that the horizon was a 

 shallow covered by warm water. Nine genera have not yet been found above 1000 

 fathoms, Ophioplinthus, Ophiernus, Ophiotrochus, Ophiocymbium, Ophiochytra, 

 Ophiambix, Ophiogeron, Ophiohelus, Ophiotliolia ; their occurrence, therefore, as fossils 

 might denote a geological bottom of great depth and covered by cold water of very heavy 

 pressure. To these might be added those species of Ophioglypha with swollen, micros- 

 copically tuberculated plates (e.g., Ophioglypha bullata), and the species of Amphiura 

 having four or five papillae on each side of the mouth angle (e.g., Amphiura patida), 

 One species only, Ophiaeantha bidentata, penetrates from the littoral zone to the lowest 

 depths. It binds together the bathymetric faunae as the humble Amphiura squamata 

 unites the geographical. Six genera, Ophiolipus, Ophioplax, Ophiobyrsa, Astroclon, 

 Astrocnida, and Astroporpa are found exclusively between 30 and 150 fathoms; five, 

 Ophiopyrgus, Ophiomastus, Ophiopyren, Op>hiocentrus, and Ophiosciasma, between 150 

 and 500 ; and four, OphiopAeura, Ophiophyllum, Astrochele, and Astroceras between 500 

 and 1000 fathoms. 



