Ixxxiv Introduction, 



the Lampreys; their single nasal sac communicates with the pha- 

 rynx, whence they are called 'Hyperotreti;' whilst in the Petromy- 

 zontidae, hence called ' Hyperoartii/ it ends blindly as in other 

 Fish. The embryos of Petromyzontidae go through a metamor- 

 phosis, being blind and edentulous when set free from the egg. 

 These larvae were formerly supposed to be a distinct species, and 

 were known under the name of Ammocoetes hranchlal'is. Like the 

 larvae of some other Fish and of some Amphibia, they may attain 

 sexual maturity whilst still in one of the stages preparatory to the 

 perfect adult condition. Tlie evolution of the sexual organs appears 

 however to exhaust the powers of such larvae as attain to it, and 

 to be incompatible with the completion of the entire curriculum of 

 metamorphosis. 



The sixth order of Fishes, the PharyngobrancJiii, are represented 

 by a single species, the Lancelot, Amphioxus lanceolatits. These 

 animals are somewhat vermiform in outline, semi-transparent, of 

 small size, being only two inches in length even when adult, 

 without either cranium or brain strictly so called, or any differen- 

 tiation of the axial notochordal, or the primitive membranous neural 

 canal. 



In this order we have pulsating vessels in the place of a saccular 

 heart, whence the name ' Leptocardia' has been given to it in con- 

 tradistinction to that of 'Pachycardia,' which expresses the condition 

 of the central organ of the circulating system of all other Verte- 

 bra ta. Another name, ' Acrania,^ indicates the fact that in corre- 

 spondence with the absence of any other encephalic nervous centre 

 beyond a dilatation in which the myelon ends, and which may be 

 considered as homologous with the medulla oblongata, no cranial 

 cavity is developed upon the anterior prolongation of the notochorJ. 

 The mouth is surrounded by a cartilaginous ring, carrying ante- 

 riorly tentacular outgrowths, whence the name ' Cirrhostomi ' has 

 been given to this order. The digestive tract immediately pos- 

 teriorly to the mouth is constituted by a multiperforate branchial 

 skeleton, along the bars of which blood is proj^elled by contractile 

 branchial arteries, and through the fissures of which the inhaled 

 water finds an exit into a cavity homologous with a branchial 

 cavity, and opening by a single orifice on the medio-ventral line, 

 posteriorly to the middle point of the animaFs length. The blood- 

 vessels, which pass from a sub-branchial vessel upwards along the 



