no THE APODID/E PARTI 



could also no doubt be equally well effected if the 

 canal were open, the increase of pressure leading 

 to a rush of water into the cavity. Further, whether 

 the canal is open at the end or closed, it would serve 

 well for enabling the animal to have a rapid perception 

 of changes of temperature in the water, the inlet of 

 cold water acting at once on the pigment cells. 



We may perhaps find some confirmation of our 

 supposition that this sensory body fulfils various 

 functions, in the fact that there are at least two kinds 

 of retinal cells. The diagrams in Fig. 27 show not 

 only differences in the forms of the cells, but also 

 different groupings of the cells. The end of the canal 

 is shown close to the posterior groups of short thick 

 cells. 



We have thus a comparatively simple but extremely 

 useful organ, probably adapted for the immediate 

 appreciation of the changes of depth and temperature 

 in the medium in which the animal lives, and further 

 for regulating its position in the water. The Apodidae 

 are, from all accounts, invariably excellent swimmers. 

 Keeping the ventral surface of the body uppermost, 

 they dive occasionally with great rapidity, rising 

 again to skim along just below the surface of the 

 water. Some organ to regulate such definite move- 

 ments is clearly necessary. 



That this is at least one of the chief functions of the 

 organ is rendered probable by its early appearance 

 in the Nauplius larva. The powerful rowing limbs of 

 this free-swimming larva render some directive body 

 necessary ; hence the appearance of this organ along 



