XIII 



PHYLUM CHORDATA 



379 



which can be isolated by cutting away the surrounding spongy 

 bone, and is then seen to form a sort of model of the contained 

 organ, to which the name bony laby- 

 rinth is applied. The . tympanic cavity 

 and columella have the same arrange- 

 ment as in the Lizard ; the narrow 

 eustachian tubes open by a common aper- 

 ture (Fig. 981, eus. ap.) in the roof of the 

 pharynx. 



Urino genital Organs. The kidneys 

 (Fig. 981, kd, Figs. 990 and 991, k) have 

 a very characteristic form. Each is a 

 flattened organ divided into three main 

 lobes and fitted closely into the hollows 

 of the pelvis. It is formed from the 

 metanephros, the large mesonephros or 

 WolfBan body, which forms the em- 

 bryonic kidney, undergoing complete 

 atrophy. The ureters (ur.) are narrow 

 tubes passing directly backwards to 

 open into the urodseum or middle com- 

 partment of the cloaca. 



The testes (Figs. 981 and 990, ts.) are 

 ovoid bodies, varying greatly in size 

 according to the season, attached by 

 peritoneum to the ventral surfaces of the anterior ends of the 

 kidneys. From the inner border of each goes off a convoluted 

 vas defer ens (vd.), which passes backwards, parallel with the 

 ureter, to open into the urodseum on the extremity of a small 

 papilla. The posterior end of the spermiduct is slightly en- 

 larged to form a vesicula seminalis (v.s.). There is no copulatory 

 organ. 



The female organs (Fig. 991) are remarkable for the more or less 

 complete atrophy of the right ovary and oviduct. The left ovary 

 (ov.} is a large organ in the adult Bird, its surface studded with 

 follicles or ovisacs, varying in size from about 15 mm. in diameter 

 downwards, and each containing a single ovum. The left oviduct 

 (I. od.) is long and convoluted ; its anterior end is enlarged to form 

 a wide, membranous cdelomic funnel (1. od.") into which the ripe ova 

 pass on their liberation from the ovisacs ; the rest of the tube has 

 thick muscular walls, lined with glandular epithelium, and opens 

 into the urodseum. A fair-sized vestige of the right oviduct (r. od.) 

 is found in connection with the right side of the cloaca, and a 

 more or less extensive vestige of the right ovary is frequently 

 present. 



Internal impregnation takes place. As the ova or " yolks ' ; 

 pass down the oviduct they are invested with the secretions of its 



FIG. 989. Columba livia. 



The right membranous laby- 

 rinth, outer aspect. FA, am- 

 pulla of posterior canal ; FB, 

 posterior canal ; HA , ampulla 

 of horizontal canal ; HB, hori- 

 zontal canal ; lag. cochlea or 

 lagena ; mr. membrane of 

 Reissner ; ph, basilar part of 

 cochlea ; S. sacculus ; >SA, am- 

 pulla of ^anterior canal ; SB, 

 anterior canal. (From Wieder- 

 sheim, after Hasse.) 



