CLASSIFICATION. 21 



of a vertebrate animal possess a tendency to arrange them- 

 selves on either side of a longitudinal axis. 



Pisces. No amnion or allantois. Posterior or visceral 

 clefts persistent, bearing branchiae* (gills) throughout life. 

 Limbs, properly so called, absent; fins median and rayed; 

 nasal sacs, as a rule, do not communicate with a mucous 

 surface; blood corpuscles nucleated and red, with exception 

 of A mphioxus (lancelet), which are colorless; heart with one 

 auricle and one ventricle, with the exception of Amphioxus; 

 skull articulates with vertebral column through single sur- 

 face, the basi-occipital bone; lower maxillary articulates with 

 skull through a chain of bones (suspensorium), and presents 

 a concave articular surface. 



Batrachia. No amnion or allantois. Posterior visceral 

 clefts generally closing in adult, so that branchiae are pres- 

 ent in the majority of cases only in the young. Limbs pres- 



* "It is at the beginning of the intestinal canal, where the ventral laminae 

 are converging, that the branchial arches are developed ; the parietes of 

 the body here become thinner ; and in this, the cervical region, several clefts 

 or fissures make their appearance, which sink downward, and penetrate 

 through the mucous layer ; there are three pairs, or, with the oval aperture, 

 four pairs of such fissures, but the posterior pair are extremely small ; they 

 are called the branchial fissures fissurae branchiales ; between them lie 

 three segments, or divisions of the ventral laminae, which are blunt and 

 rounded anteriorly, beveled off toward the digestive cavity, and therefore 

 sickle-shaped; these are named the branchial arches arcus branchiales; 

 the fourth branchial arch is placed hindmost, and is not yet distinct from 

 the ventral lamina. On the fourth day the two most anterior branchial 

 arches increase in thickness; a new fissure is formed posteriorly; on the 

 fifth day the foremost fissure closes, and the foremost branchial arch unites 

 with its fellow of the opposite side, and forms the lower jaw ; the next in 

 .succession. is transformed into the os hyoides. The two last branchial fis- 

 sures close upon the fifth day; at the same time the first is lost entirely; 

 but the second continues longer open. On the third and fourth days the 

 part of the ventral lamina, which is situated in front of the lower jaw, thick- 

 ens, and resolves itself into the lower jaw ; this part is more strongly marked 

 on the fifth day. The two sides of the upper jaw do not meet in the first 

 instance ; they coalesce at a later period, through the medium of the frontal 

 process which is developed betwixt the eye." Extract from chapter on 

 Embryology in Wright's trans, of Wagner. 



