456 Causes and Colrse of Organic Evolution 



pp. 175-76), one median and two paired organs may arise, 

 of which the median persists in higher types. The paired 

 glands, in relation to the frontal organ or nasal sac, seem to 

 correspond with the accessory nasal glands met with in verte- 

 brates from the Amphibia upwards. The frontal organ in 

 nemerteans is at times prolonged downward and backward 

 as a richly glandular structure, whose posterior part is on a 

 level with the nerve ganglia of the brain, also with the front 

 part of the proboscis sheath and of the pharynx. The rela- 

 tion of these structures in Prosadenoporus suggests that the pos- 

 terior glandular part of the frontal organ might readily in 

 course of evolution open into the pharyngeal region, and so 

 though still unpaired seems to correspond exactly in position, 

 relation, and development to the organs of Jacobson soon to 

 be mentioned. 



In Amphioxus the median ciliated "olfactory pit" evidently 

 corresponds to the frontal organ just described, though it 

 shows secondary changes of relation. 



In Cyclostomata the nasal organ is, like the frontal organ 

 of nemerteans, a single median dorsal involution on the anterior 

 end of the snout, and like it consists of an anterior orifice, a 

 canal, a nasal sac or pouch, and a down-growing process that 

 in the lamprey as in nemerteans ends blindly, but in Myxine 

 is prolonged downward to open "into the oral cavity, perfor- 

 ating the skull from above, instead of from below as in other 

 vertebrates" {138: 198). In cyclostomes the single nasal sac 

 becomes either partially or completely divided by a median 

 partition; and the ciliated lining tissue becomes raised into 

 radial ridges. On either side of the nasal sac patches of gland- 

 ular tissue arise, that very probably represent the head glands 

 of nemerteans. But in Bdellostoma, according to Dean, the 

 internal division of the nasal sac seen in Petromyzon and Myx- 

 ine becomes complete, since in it the olfactory organ arises as 

 a pair of ingrowths. 



In Batrachia Apoda the nasal organ has now become paired 

 throughout, but still occupies the anterior dorsal region of 

 the snout. Each passage opens posteriorly into the oral cavity 

 as in Myxine, and so a possible respiratory relation is opened 

 for these, specially if they become land forms in the adult 

 state, as most now are. Lateral glandular outgrowths are 

 also formed ventrally here, that in some Apoda communicate 

 with the nasal sacs, in others l)ecome shut off from the nasal 

 sac, though innervated by the olfactory nerve. These form 

 the organ of Jacobson, that seems to have a chemotactic func- 

 tion, and that persists even up to man. As in cyclostomes 



