4o8 CYCLOSTOMATA. 



DIAGRAM' XXV. 



Cyclostomata. 



The figure named Lamprey (after Nuhn) shows from the ventral 

 surface the seven gill-pouches (^.^.). opening directly to the exterior 

 {g. a.), opening internally into a respiratory tube {r. t.), which lies ven- 

 tral to the gullet. The dotted lines {g.) indicate the posterior contin- 

 uation of the gullet, which is hidden anteriorly by the respiratory tube. 

 The horny teeth {(.) in the mouth are shown. 



The figures named Gullet (after Wiedersheim) show the relation of 

 the gullet and the respiratory tube in the lamprey. The upper figure 

 refers to a larva in which there is no distinction between gullet and 

 respiratory tube. The lower figure refers to a metamorphosed larva, in 

 which the respiratory tube [r. t.) is distinct from the dorsal gullet (d. g. ). 

 At the foot of the page (larva) is a longitudinal section of a larval lamprey 

 (after Balfour). The invagination forming the mouth {m.), the invagi- 

 nation forming the nostril (o/.), the gill-pouches {g. p.), the ventral con- 

 striction which forms the thyroid (th.), the heart [ht.), the ventral aorta 

 (v. ao.), the notochord (notoch.), the brain (br.), the auditory vesicle 

 {au. V. ), are shown. 



The figure named Hag, represents diagrammatically some of the 

 structures of Myxine : — The nostril (». ), the posterior opening into the 

 mouth (/. K.), the tongue muscle (t) thrown to the side, the tooth- 

 plates (t. pi.), the gullet {g.), the gill-pouches {g. p.), the efferent bran- 

 chial tubes (e. br. t. ), the convergence of these tubes to a single aperture 

 on each side, an unpaired duct from the gullet to the aperture on the 

 left side (</. br. ce.), the position of the heart {ht.), the anterior ends of 

 the segmental ducts (j. d. ). 



Sect, is a diagrammatic section of the hag in the region behind the 

 heart, showing : — The spinal cord {sp. c), the notochord (».), the lateral 

 muscles, the gut {g.) with internal ridges, the segmental ducts (s. d.), 

 with an artery and a vein between them, the pores (p.) of the lateral 

 glands. 



The figure named Seg. duct (from Gegenbaur, after MuUer), shows a 

 magnified part of the segmental duct (s. d. ), with lateral nephridia, and 

 associated blood-vessels {gl.). 



