iv. i7 



AMMOCOETE LARVA 

 r 2 3 



H5 



Fig. 78. Development of the endostyle of the lamprey. Sagittal sections through 



the head at three stages. 



1, auditory sac; 2, medullary tube; 3, myotome; 4, conus arteriosus; 5, endostyle; 6, first 



gill-slit; 7, first arterial arch; 8, notochord; 9, inpushings which cut off the endostyle from 



the pharynx; 10, aorta; 11, stomodaeum. (After Dohrn, from Kukenthal.) 



bottom they rapidly disappear and remain hidden indefinitely, the 

 heads perhaps just visible in small depressions made by the rhythmic 

 respiratory movements. When disturbed they always swim with the 

 head downwards and in contact when possible with the ground. This 

 habit leads them to burrow rapidly. It is not known whether they 

 have other receptors to guide them to mud rich in possible food 

 organisms. The nasal and hypophysial sacs are poorly developed in the 

 larva, and the sense of smell can hardly serve this purpose. 



