460 E. EAY LANKESTER AND AETHDE WILLEY. 



median line, it is clear that the anterior slits must undergo a 

 translation in growth which moves them up the right side. 

 Now^ if we look at the slits following No. 6, it appears as 

 though a translation of these hinder slits were in progress^ 

 tending to bring them into position on the left side when 

 fully formed. We do not, however, consider it likely that 

 such a movement of the hinder slits to the auimaPs left side 

 takes place, but believe that they also in due time move up 

 firstly to the right side, alongside of those in front of them. 

 We have found it impossible with our present material to trace 

 the immediately subsequent history (subsequent to the stage 

 drawn in figs. 4 and 6) of the gill-slits. We are of opinion 

 that Kowalevsky's very definite statement and figures given in 

 the ' Memoires de FAcad. Imp. de St. Petersbourg,' 7th series, 

 vol. xvi. No. 12, 1866, must be accepted. According to that 

 account, after some dozen gill-slits have taken up their position 

 on the animal's right side — having moved into that position 

 from the median line — a new and startling change occurs. 

 The whole series moves downwards across the median line and 

 up the left side of the pharynx, so that the primitive right-side 

 gill-slits become the left-side series ; and in the meanwhile a 

 new series corresponding in number make their appearance 

 not one by one, but all together, in the right side of the 

 pharynx, occupying, as it were, the position deserted by the 

 rotated primitive series. This movement of growth appears to 

 be a general one affecting the whole pharynx, for, simulta- 

 neously with the translation of the primitive gill-slits from 

 right to left, the great larval mouth moves from its extra- 

 ordinary position on the animal's left side, and, becoming 

 relatively very much smaller, takes up its permanent position 

 as an anterior median orifice whilst its hood and tentacles 

 appear. We have not, we regret to say, at present been able 

 to study any larvse in which these remarkable changes are in 

 progress. We have, however, many larvse in which they are 

 completed. It is noteworthy that these larvse are scarcely, if 

 at all, larger than that of PI. XXIX, fig, 6 ; and yet they have 

 the mouth reduced in size and nearly median in position, the 



