1 6 Pratt, Bipolar Theory. 



range of distribution is dependent to a great extent upon 

 the length of the larval free -swimming stage, which we 

 have reason to believe lasts only for a very short period ; 

 it is, therefore, hardly possible that this form could cross 

 the Atlantic. 



A tropical species of this genus, A. cristata, has been 

 recorded from Jamaica, but is quite distinct from the 

 species under discussion. Thus it would appear that A. 

 claparedii is extra-tropical in its range of distribution, 

 (iii.) Polydora polybrancJiia. 



Distribution. Shallow water. Straits of Magellan, 

 Sydne}', Naples, English Channel, Falkland Islands. The 

 genus has been taken in fairly cool and warm waters of 

 the northern and southern temperate zone, but up to the 

 present has not been taken in the tropics. 



The species appears to be strictly extra-tropical in its 

 distribution, but as it has been found in water-logged 

 timber, we must regard it as doubtfully a " bipolar form.'' 



The genus contains many sand-dwelling species, which 

 are confined to extra-tropical waters, and therefore may 

 be regarded as truly " extra-tropical." 

 (iv.) Bispira niarice? 



It is not certain that this form is identical with the 

 Mediterranean form.'* New to Falkland Islands. 



The genus has been taken in the English Channel and 

 in the Mediterranean, but this appears to be the first 

 record of the genus from the southern hemisphere, .so that 

 the genus and species appear to be both extra-tropical. 



Ortmann ('99, p. 590) says : — " Two cases of bipolarity 

 of species and one of genera have been discovered, and 

 when wc add to these the single case previously established 

 {Crangoii) we have altogether four cases of true bipolarity 

 which are to be explained by a theory." 



* Cf. Manchester Memoirs^ Vol. 45, No. 13, p. 15. 



