160 



Proccediiifjs of the Royal Physical Society. 



The following table shows the relative length (in milli- 

 metres) of the head, pectoral and ventral fins, in larv?e of 

 varions sizes, to which the measurements in the adult of 

 M. cimhria and in very early embryos of M. mustela are added 

 for comparison : — 



It will be seen from the table that the ventral fins must 

 undergo an exceedingly rapid development shortly after the 

 embryo is hatched. In Motella mustela at the time of 

 hatching the ventrals are only recognisable as a faint ridge. 

 Four or five days afterwards they are about "12 mm. 

 long, and form simple, oval, deeply-pigmented organs, which 

 are apparently still without rays. In the smallest larvoe of 

 M. cimhria which have come under my notice, the ventrals 

 are relatively very large, and have a length equal to a third 

 of that of the whole embryo. They form fan-like organs 

 supported by four stout rays, all of which are similar in 

 length. At this stage the relative length of the ventrals is 

 OTeatest, and in later stafijes the fins become reduced in 

 breadth, whilst at the same time the rays are relatively more 



1 The lenf^th of the newly-liatched embryo of Motella mustela is incorrectly 

 given as 2-25 mm. in my ])npt'r on that species already referred to. The 

 measurements taken at the time show a variation between 2-118 and 

 217 mm. 



