238 



VERTEBRATE ANIMALS. 



senting the fore and hind Hmbs. The mouth is a longitudinal 

 fissure, placed at the front of the head, and completely destitute of 

 jaws, but surrounded by a number of cartilaginous filaments. The 

 throat is provided with several leaf-like filaments which are i-ichly 

 supplied with blood, and are believed to discharge in part the function 

 of gills. The mouth (fig. 171, o) opens into a dilated chamber, which 

 is considered to represent the pharynx, and is termed the pharyngeal 

 or " branchial " sac. The walls of this chamber (6) are strengthened 

 by numerous cartilaginous filaments, between which is a series of 

 transverse slits or clefts, and the whole is covered with a richly cili- 

 ated mucous membrane. The function of this sac is cleaiiy respira- 

 tory, the water from without being admitted through the mouth, 

 passing through the branchial clefts into the abdominal cavitj', and 

 finally escaping by means of an aperture placed on the ventral sur- 

 face a little in front of the anus. From the hinder end of the 

 branchial sac proceeds the alimentaiy canal, which has appended to 



P'ig. 171. — The Lancelet (Amphioxus UtTiceolatvs), enh-irged to twice its natural size. 

 Mouth ; & Branchial .'*ac ; g Stomach : li Diverticulum rejiresenting the liver ; 

 i Intestine ; u Anus ; n Notochord ; / Rudiments of fin-rays ; p Ahdominal i)ore. 



it a sac - like organ believed to represent the liver, and which 

 terminates behind in a distinct anal aperture. There is no heart, 

 and the circulation is entirely effected by means of several con- 

 tractile dilatations, developed uj)on the great blood-vessels. The 

 blood itself is colourless. No kidneys have hitherto been dis- 

 cdvered, and the reproductive elements are emitted into the abdom- 

 inal cavity, from which they escape by the pore placed upon the 

 lower surface. 



There is no skeleton properly so called. The notochord (») 

 remains throughout life as a semi - gelatinous rod, enclosed in a 

 memliranous sheath, and supporting the spinal cord. There is no 

 skull, and the spinal cord does not expand in front to form a dis- 

 tinct brain or enrephalon. The brain, however, may be said to be 

 represented, as the front portion of the nervous axis gives off nerves 

 to a pair of eyes, and another branch to a ciliated pit, which is be- 

 lieved to be a rudimentary organ of smell. 



