542 



CHORDATA. 



be added. It arises from the inner angle of the eye, and can extend over 

 the cornea beneath the upper and lower lids. A special lachrymal gland, 

 which occurs at the outer angle of the eye, provides the fluid to moisten, 

 the cornea, while a second or Hurder's gland occurs at the inner angle^ 

 when a nictitating membrane is present. Both are lacking in the An- 

 ainnia. 



The ear, at the level of the medulla oblongata, rivals the eye in 

 its complication of structure. In development it has one point in 



common with the invertebrate 

 ofcocyst it 

 dermal pit 



ass 



ecu 



arises as an ecto- 

 which is usually 

 completely cut off from its par- 

 ent layer, and only in elasmo- 

 branchs remains connected with 

 the exterior by a tube, the 

 elsewhere closed endolymphatic 

 duct. In the cyclostomes it con- 

 sists of a single vesicle with a 

 single macula acustica; from 

 the fishes upwards the vesicle 

 becomes divided by a constric- 

 tion into an upper utriculus 

 and a lower sacculus (fig. 575), 

 the connecting utriculosaccular 



FIG. 575. Diagram of membranous laby- duct being narrow ill the mam- 



rinth of a fish. (From Wiederaheim.) 

 aa, ae, ap, anterior, external, and poste- maiS. 

 rior ampullae ; ass, superior utricular 

 sinus ; ca, ce, cp, anterior, external, and 

 posterior semicircular canals ; cus, utri- 

 culosaccular canal; de, ductus en- 

 dolymphaticus ; 7, lagena ; rec, recessus 

 utriculi ; se, sacculus utriculi ; ss, supe- 

 rior utricular sinus ; sp, posterior utri- 

 cular sinus; u, utriculus; t, origin of en- 

 dolymph duct. 



culus 

 macula 

 from t 



Both utriculus and sac- 

 receive a part of the 



a 



-i. 



acustica. Diverticula 

 occur, giving 

 name of labyrinth. 

 From the utriculus arise three 

 semicircular canals, connected at either end with this cavity, each 

 swollen at one end to an ampulla, containing a special nerve 

 termination, the crista acustica. These canals stand at right 

 angles to each other in the three dimensions of space and with- 

 out doubt subserve the sensation of equilibration (p. 128). They 

 are an outer horizontal, an anterior vertical (nearly sagittal), and 

 a posterior vertical (nearly transverse). The non-ampullar ends 

 of the two vertical canals unite, a condition which is understood 

 when it is recalled that in cyclostomes these canals alone are 

 present, and in Myxine form a single canal with two ampullae. 

 A later formation is a diverticulum from the sacculus, which 



