85 



The larvae may therefore be said to be prevalent during a 

 period of about three months, but mostly in July and August. At 

 St. Andrews it may be said to occur inshore, certainly well within 

 the bay ; on the Northumberland coast it is never found in any 

 of the bays, but well out from the coast, except at the north end 

 of the countj^, in the neighbourhood of the Fame Islands, where 

 it has been found nearer the coast. 



This all goes to show that in the North Sea the larvae are liable 

 to a period of denatation, a drift taking place which serves to unite 

 the whole of the east coast of Scotland and Northumberland with 

 the eastern coast of the North Sea. It may be taken for granted 

 that wherever metamorphosis has been seen to take place 

 commonly there also adults are developed. This has been noted 

 at Shetland, St. Andrews and Heligoland. There are i)robably 

 other centres of dispersal which lie along the path of the ocean 

 current, and the larvae are thus exposed to a dispersal which must 

 take place to the south along the east coast of Britain, ultimately 

 reaching Heligoland, and the south-west coast of Norway. The 

 larva has been found at Bergen, and rarely at Kiel. The varia- 

 bihty in the number of larvae in successive years is evidently 

 associated with this denatation. 



The Shetland, St. Andrews and Northumberland specimens 

 all agree with the form originally described by Mliller and by 

 Wagener. Much variation is met with affecting the size, the pig- 

 ment, the size and number of the tentacles, the shape and size of 

 the anal ring and so on, but when large numbers of specimens 

 are examined it is found that seldom do all the variations occur 

 together, some feature indicates definitely the specific type. 



No other species of the larva, except Actinotrocha branchiata, 

 has been found on the east coast of Britain, and no other species 

 of the adult than Plioronis ovalis, and we are justified for this reason 

 in associating them as the larva and the adult of the same species. 

 Harmer in his paper is dis^^osed to think as he found Plioronis 

 ovalis multiplying freely by transverse fission " that all the 

 observed individuals may well have been produced in this way." 

 He forgets, however, that the specimens be examined presented 

 fully developed gonads. We would thus have the paradox that 

 in the same area there is an Actinotrocha the adult of which has 

 never been seen, and a Phoronis which does not }ield a larva or 

 yields a larva which has not yet been found. 



