KTTNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 47. NIO I. 1 7, 



and at different scasons I have made repeated attempts to find larvse - bu1 always 

 without result. ()n the other hand, I have dredged very small young from a bottom 



of mud. 



Considering that the Priapulids in thc adult state are incapable of any notice- 

 able removal and that seemingly their young are boundinthe same way and, höides. 1 hat 

 hitherto not a single specimen of a Priapulus has been found under the subeqnatoreal 



and equatoreal zones, how then interpret the fact, that representatives of this 'jenus 

 are living in both the cold seas, but entirely absent in the intermediate torrid zones? 

 In view of the arguments given above, I maintain the following statements 

 with regard to shallow-water animals: 



1) That bipolär animals most closély relaied to each oiher exist, though neither in 

 the adult nor in the young stages are they ever capable of any perceptible removal, and 

 are never found living in the intervening regions at the present time. 



2) That this relation in many or most instances cannot be due to a convergence- 

 phenomenon alone. 



3) That an exchange of adult animals from polar-circle to polar-circle is quite 

 out of the question. 



4) That larvce of shallow-water animals are incapable of sustaining the very long 

 transport from polar-circle to polar-circle. 



5) That the floating-period of marine larvce is very limited. 



6) That under-water currents uniting the two polar seas, even if they rim in a 

 straight direction, run so very slowly that they cause the destruetion of the larvce. 



7) That the phenomenon of a true bipolarity requires a specific interpretation. 



As mentioned above, in working with the Holothurioidea of the Challanger 

 expedition, I had an excellent opportunity of observing the astonishing similarity 

 between northern and southern forms of this group, though I never saw a single 

 species common to both seas. 



In view of what was mentioned at the beginning of this paper, the represent- 

 atives of the genus Priapulus or of its progenitors must have had a world-wide 

 distribution during a past period, but for several reasons, e. g. altered physical con- 

 ditions, they have become extinct under the tropical and temperate zones, but con- 

 tinued to exist in the cold-water regions, where the more uniform physical conditions 

 produced descendants which developed slowly and after almost the same plan. 

 According to this view, the northern Priapulus caudatus Lam. and the southern 

 Priapulus caudatus forma tuberculato-spinosus Baied are »relicta» of the same proge- 

 nitors, which at an epoch of the evolution of the globe had a world-wide distribution.. 



This is the only interpretation which I can give with regard to the bipolarity 

 of the two forms of Priapulus, absent in all intermediate zones, devoid of a power 

 of removal of their own in any noticeable sense, and evidently in want of free 

 larval stages. 



But, further, I also broached the opinion that the descendants of the progenitors 

 in question, if they were able to sustain the altered physical conditions under the 

 tropic and temperate zones and to adhere there, must get changed in their organi- 



