

Marine Fauna of Great Britain. 411 



line in all parts of the world. In temperate zones, where there 

 is a wide range in annual temperature, the pelagic larvae of 

 benthonic animals appear only in the spring and summer 

 seasons ; in the tropics they are present at all times in the 

 surface-waters. If there were once a nearly universal climate 

 over the whole ocean, we may suppose that the same species 

 of benthonic animals were nearly everywhere present in the 

 shallow-water zones. When cooling at the poles set in, those 

 animals with pelagic larvae would be killed out or be forced 

 to migrate towards the warmer tropics. By being able to 

 limit the reproductive process to the summer season, some of 

 these organisms with free-swimming larv^ have been able to 

 live on in the temperate regions, but in the tropical and coral- 

 reef regions we have the remnants of a once universally dis- 

 tributed shallow-water fauna. With the disappearance of 

 this shallow-water fauna from the polar regions its place 

 would be occupied by the organisms from the deeper mud- 

 line, very few of which possess pelagic larvae." 



With respect to the first part of this quotation it is doubtful 

 if the actual facts connected with the polar fauna bear out this 

 interpretation. In the first place, well-known deep-sea (or, as he 

 calls them, benthonic) arctic animals have pelagic larvaj, such 

 as sponges, zoophytes, star-fishes, and annelids. The condi- 

 tion of the latter alone would prove fatal to the argument. 

 Further, the mere examination of surface-organisms in the 

 tow-nets is no proof that swarms of pelagic larvge do not 

 exist near the bottom. The pelagic larva3 of the pteropods 

 and hydromedusse that abound in the " whale-food," and the 

 pelagic eggs and larvge of fishes, would be quite as likely to 

 suflfer as the ciliated young of the benthonic forms previ- 

 ously mentioned. Again, while it is true that certain polar 

 forms have hollows or pouches in which the young are reared, 

 tlje same holds good with Asterias Mulleri and Cribrella 

 sanguinolenta of the tidal rocks, and the Autolytus of the 

 inshore waters of St. Andrews. The number of these forms 

 in the polar waters, moreover, is out of proportion to those 

 which have no such provision, and in which the larvee are 

 free-swimming. So far as present knowledge goes the same 

 remarks apply to " animals living about and deeper than the 

 mud-line in all parts of the world." 



Some modification is also necessary in regard to the 

 statement that " In temperate zones .... the pelagic larvae 

 of benthonic animals appear only in the spring and summer 

 seasons," whereas " in the tropics they are present at all 

 times at the surface." Now in climates like our own it is 

 well known that delicate pelagic forms like ciliated larvse 

 prefer the surface only in mild and calm weather, but their 



