PLANKTON 257 



appeared suddenly in our seas, there is no doubt that it was 

 not present in the Irish Sea plankton gatherings previous 

 to 1909, but has been abundant since that year, and has 

 completely adopted the habits of its English relations — 

 appearing with B. mobiliensis in late autumn, persisting 

 during the winter, reaching a maximum in spring, and dying 

 out before summer. 



The NaupUus and Cypris stages of Balanus in the plankton 

 form an Interesting study. The adult barnacles are present 

 in enormous abundance on the rocks round the coast, and 

 they reproduce in winter, at the beginning of the year. The 

 newly emitted young (Nauplii) are sometimes so abundant 

 as to make the water in the shore pools and in the sea close 

 to shore appear muddy. The Nauplii (Fig. 2 on Plate XXII) 

 first appeared at Port Erin, in 1907, in the bay gatherings on 

 February 22 (in 1908 on February 13), and increased with 

 ups and downs to their maximimi on April 15, and then 

 decreased until their disappearance on April 26. None were 

 taken at any other time of the year. The Cypris stage (Fig. 3 

 on Plate XXII) follows on after the Nauphus. It was first taken 

 in the bay on April 6, rose to its maximum on the same day 

 with the Nauplii, and was last caught on May 24. Through- 

 out, the Cypris curve keeps below that of the Nauphus, the 

 maxima being 1,740 and 10,500 respectively. Probably the 

 difference between the two curves represents roughly the 

 death-rate of Balanus during the Nauphus stage. That 

 conclusion I think we are justified in drawing, but I would 

 not venture to use the result of any haul, or the average of 

 a number of hauls, to multiply by the number of square 

 yards in a zone round the coast in order to obtain an 

 estimate of the number of young barnacles, or, after a further 

 calculation, of the old barnacles that produced them — 

 the irregularities are too great. 



To my mind it seems clear that there must be three factors 

 making for irregularity in the distribution in space and time 

 of a plankton organism : — 



s 



