434 THE ANIMAL KINGDOM. 



the two anterior lower labials seen in the Anuran Tadpole. There are also 

 three minute lower labial cartilages paired right and left. The upper and 

 lower barbules of the Myxinoidei are supported by cartilaginous rods, but 

 there are no labial cartilages. The Lampreys have a complicated carti- 

 laginous framework supporting the branchiae and the heart. It lies super- 

 ficially, and must be compared with the extra-branchial cartilages of 

 Anuran Tadpoles and Sharks. The notochord is large, with a tough 

 fibrous sheath. There are small cartilaginous neural arches in the Lam- 

 prey, but not in Myxinoidei^ and in the former two cartilaginous rods are 

 applied to the ventro-lateral aspect of the notochord in its whole extent. 

 Cartilaginous rays support the azygos fins. 



The brain has the usual parts, but in the larval Lamprey the thalami 

 optici and mid-brain are scarcely separated and are elongated, as is also 

 the medulla oblongata. There are two olfactory lobes, and the cerebellum 

 is a simple transverse commissure. The cerebral hemispheres are solid in 

 the Myxinoidei. The spinal cord is flattened dorso-ventrally. The anterior 

 and posterior roots of the spinal nerves alternate with one another, and do 

 not unite in the Lampreys, but they both divide into dorsal and ventral 

 branches. In Bdellostoma and Myxine there are two anterior roots to 

 every posterior root throughout a portion of the cord at least. Their 

 ventral branches unite, and not their dorsal ; but the dorsal branches 

 derived from each pair of anterior roots fuse together. There is no 

 sympathetic system, but the intestinal branches of the vagus extend 

 nearly the whole length of the digestive tract. The nostril is single, even 

 in development, though the olfactory lobes are double. It is tubular, and 

 the tube is supported by cartilage, which in the Myxinoidei forms a series 

 of rings. A passage leads from it to the pharynx, which is blind in the 

 Lamprey, but opens in the Myxinoidei in front of the velum. The eye in 

 the latter wants the eye-muscles, the sclerotic, the iris and lens. In the 

 larval Lamprey (Ammocoetes) it remains beneath the epidermis, derm and 

 subdermic tissues, which represent the cornea ; there is no iris, and the 

 lens retains the embryonic structure. At the metamorphosis the eye 

 travels to the surface, and becomes fully developed. The ear in the Hag- 

 fish consists of a vestibule and single semicircular canal. In the Lamprey 

 there are two vertical semicircular canals, with ampullae as usual, and 

 indications of cochlear and saccular outgrowths. There are motile cilia in 

 the vestibule, as in the otolithic vesicles of Mollusca, &c. 



The mouth of the adult Lamprey is concave, suctorial, fringed by a soft 

 lip bearing numerous short filamentous processes, and armed with numerous 

 epidermic teeth. In the Myxinoidei it has barbules above and below, and 

 the teeth are reduced to a single upper tooth and to two rows borne by the 

 supra-lingual cartilage. The Ammocoetes has a hood-shaped mouth with a 

 fringe of tentacles. The tongue (so called) is of immense size, especially in 



