1897.] PLANKTON OF THE FAEEOE CHANNEL. 805 



it seemed to me worth while to study all the stages in my power, 

 and to endeavour to put the matter straight again. 



The following stages have been drawn from ; Eesearch ' speci- 

 mens and cut into microscopic sections : 



Stage. Tentacles. Mesenteries. Oral tentacles. 



A 6 10 



B 7 11 4 



Appearance of the unpaired tentacle (5). 



C 9 12 4 



D 9 12 4 



E 10 14 6 



F 11 16 8 



a 12 16 8 



H 13 18 8 



I ? 19 10 



(First appearance of generative cells.) 

 A few older stages have also been studied. 



This table, taken together with the diagram (PL XL VII. fig. 1), 

 sufficiently shows the successive development of the various struc- 

 tures, and their position in the oldest specimens. As regards this 

 diagram, the order of succession of the first four pairs of mesenteries 

 is taken from van Beneden's account of an allied species, and that 

 of the first two pairs of tentacles is inferred from his drawings and 

 descriptions. The facts implied by the remainder of the diagram 

 I have myself checked, and they will be found to differ entirely 

 from those given by Vanhoffen, and to agree with those of Boveri 

 on all points with which we both deal. The developmental order of 

 the first four pairs of mesenteries, as described by van Beneden (c, 

 a, 5, cZ), appeared at first to contradict the lettering attached to 

 the same mesenteries by Boveri (c/, a, 6, c), but the latter author 

 courteously informs me that he did not intend by these letters to 

 indicate a developmental succession : van Beneden's observed 

 order may therefore be taken to hold good for this species also, in 

 default of direct evidence. 



AEACHNACTIS BOURNEI, sp. n. 



There can be no doubt that the specimens from the English 

 Channel, first recorded by Bourne, and described by van Beneden, 

 tinder the name of Amchnactis albida, belong to another and 

 an unnamed species. Not only are the form, and proportions of 

 the animal quite different from those of albida, both in van 

 Beneden's drawings and in a few specimens which I received from 

 the Marine Biological Station at Plymouth in 1893, but also the 

 rate at which different sets of organs are developed is not the 

 same in the two species. This is at once apparent on a comparison 

 of my table of albida stages (given above) with the following : 



Tentacles. Mesenteries.. . C ? ra ! 



tentacles. 



Van Beneden's oldest larva . . 7 8 



Plymouth specimens (1893) . . 9 10 2 



PROC. ZOOL. Soc. 1897, Xo. LIIL 53 



