400 ZOOLOGY FOR MEDICAL STUDENTS chap. 



other — the name pineal being given to the posterior one while the other 

 is termed the parapineal or parietal organ. Each of these forms an im- 

 perfect eye-hke organ, its superficial wall being clear and transparent 

 and its deep wall being retinal in structure, with rods projecting into 

 the cavity of the organ and with nerve-fibres passing from it away 

 towards the brain. 



The Myxinoid brain is remarkably different from that of the Lampreys : 

 it is much thicker as compared with the spinal cord and much shorter : 

 in both aspects it is less primitive than that of the Lamprey. The 

 pineal organs are much less developed and there is no trace of eye-like 

 structure. 



The pituitary organ of the Cyclostomes is remarkable for the fact 

 that it retains throughout Hfe its opening on the outer surface of the 



Fig. 176. 



The head and anterior part of the body of Bdellostimia cut in the sagittal plane to show the relations 

 of pituitary tube and buccal cavity. b.c, Buccal cavity ; br, brain ; N, notochord ; olf, olfactory 

 organ ; p.o, pituitary opening ; p.t, pituitary tube ; ^, teeth ; vx, internal openings of gill-sacs. 



skin. In Bdellostoma the opening is situated at the tip of the head 

 (Fig. 176, p.o) but in the Lampreys it becomes, during the course of 

 development, shifted in a dorsalward direction until it comes to be 

 situated right up on the dorsal side of the head — a considerable distance 

 from the front end (Fig. 175, p.o). In the Lampreys the pituitary organ 

 retains its blind end posteriorly but in the Myxinoids it comes to open 

 into the posterior portion of the buccal cavity (Fig. 176), forming a 

 channel through which the creature takes in the water for respiration. 



The great organs of sense have marked peculiarities. The olfactory 

 organs become involved in the ingrowth of ectoderm which gives rise 

 to the pituitary organ and are carried in with it, so that in the adult 

 the olfactory organ (the right and left being fused together) forms simply 

 a kind of pocket on the dorsal or hinder wall of the pituitary tube a little 

 way within its external opening. The eyes in the Myxinoids never 

 complete their development : they remain sunk beneath the skin as 

 more or less embryonic non-functional rudiments. The otocysts also 



