39 8 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



lying underneath the pharynx and into which the gland cells continue to 

 pour their secretion. The endostyle has become converted into a ductless 

 -land and that ductless gland is clearly recognizable as the thyroid. 

 Here then by the development of the Lamprey we have light thrown 

 upon the evolutionary history of this organ. The thyroid gland of the 

 Yertel>rata would appear to be the persistent and modified endostyle of 

 the ancestral vertebrates in which the secretion was used in connexion 

 with feeding. 



As regards the remainder of the digestive tube all that need be said 

 is that it is relatively very simple and undifferentiated. In the ordinary 



FIG. 175. 



Sagittal section through head region of a Lamprey (Petromyzon). a.c, 41imentary canal ; 

 b.c t buccal cavity ; br, brain ; N, notochord ; olf, olfactory organ ; p.o, pituitary opening ; p.t, pituitary 

 tube ; r.t, respiratory tube ; s.c, spinal cord ; t, tongue ; V, ventricle ; v, velum ; v.a, ventral aorta ; 

 v.c, internal openings of gill-sacs. 



River Lamprey it becomes partially degenerate and non-functional in 

 the adult. 



The coelome shows in the Myxinoids the unique peculiarity that 

 there is no demarcation of pericardiac and peritoneal cavities. 



Tlu-. renal organ of the adult Cyclostome consists of an elongated 



opisthonephros which, however, in the Myxinoids shows a remarkably 



.simplified condition the individual tubules having become greatly 



thickened and shortened, so that instead of being intermingled to form 



a compact kidney they stand out perfectly distinct, appearing as short 



wide- ilivi-rtirula from the longitudinal duct. The ovary or testis is 



lc and sheds its gametes into the splanchnocoele whence they pass 



i minute opening (genital pore) through the side wall of the urino- 



ul sinus into its cavity and thence to the exterior. The morpho- 



il meaning of these genital pores is obscure but they may perhaps 



be regarded as Miillerian ducts which have gradually become shorter 



