398 



ZOOLOGY FOR MEDICAL STUDENTS 



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

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

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

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

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

 Vertebrata 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 



■p.o. olf hr 



K 



Fig. 175. 



Sagittal section through head regioii of a Lamprey ^Petromyzon). a.c. Alimentary canal \ 

 hj:, buccal cavity ; hr, brain ; JV, notocbord ; oi/, olfactory organ ',pM, pituitary opening '^ p.t, pituitary 

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

 v.Cf 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. 



The renal organ of the adult Cyclostome consists of an elongated 

 opisthonephros which, however, in the Myxinoids shows a remarkably 

 simpUfied 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 diverticula from the longitudinal duct. The ovary or testis is 

 single and sheds its gametes into the splanchnocoele whence they pass 

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

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

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

 best be regarded as Miillerian ducts which have gradually become shorter 



