362 ANIMAL BIOLOGY. 



paired excretory organs. The group is a large and diverse one, 

 including the Platylielminthes (liver-fluke and tapeworm), in 

 which the body is flattened, and is either unsegmented (liver- 

 fluke) or divided into a number of successive divisions, which 

 have a tendency to separate individualisation (tapeworm), 

 without a ventral nerve-chain, generally hermaphrodite, without 

 lateral appendages, but provided with suckers (liver-fluke) or 

 suckers and hooks (tape-worm) ; and the Cho&topoda, ringed 

 worms (Annelids), possessed of a ventral nerve-chain and setose 

 locomotive appendages, which may be numerous in each seg- 

 ment (Pofychceta), or, as in the case of the earth-worm, few 

 (Oligochoeta). 



(3) The Mollusca, bilaterally symmetrical (mussel) or some- 

 what asymmetrical (snail) animals, without lateral appendages, 

 but possessed of a muscular locomotive 'foot/ unsegmented in 

 the adult form, and with paired ganglia united by commissures 

 which form an cesophageal nerve-ring,, generally possessing a 

 bivalve (mussel) or univalve (snail) shell, and a bilobed (mussel) 

 or unilobed (snail) mantle. To this group belong the mussel, 

 which has plate-like gills (Lamettibranchiata), no separate head, 

 and in which the sexes are distinct ; and the snail (Gastropoda), 

 in which the head is distinct, and the mouth is provided with 

 an odontophore (Odontophora), having a twisted univalve shell, 

 and pulmonary respiration (Pulmonata). 



The Vertebrata, are bilaterally symmetrical animals, with a 

 notochord, and in nearly all cases an internal skeleton, the 

 vertebral column, and a dorsal nerve system generally protected 

 by the arches of the vertebrae. There are never more than two 

 pairs of limbs, and the jaws, which are never (as in the arthro- 

 pods) modified appendages, move up and down. They are 

 always possessed, at some period of life, of visceral arches and 

 clefts. They are divided into three great groups. 



1. The Ichthyopsida, including the Pisces (cod) and the Am- 

 phibia (frog). Eespiration by gills always takes place during a 

 part (frog), and sometimes the whole of life (cod). The exoskele- 

 ton is either absent or very slightly represented. The basi- 

 occipital region of the skull is generally unossified. The 

 temperature of the blood varies with, and is not much above, 

 that of the surrounding medium. The red blood corpuscles are 

 nucleated. There are two systemic aortic arches. The urinary 

 organs are persistent Wolffian bodies. There is no thoracic 

 diaphragm. There is no true corpus callosum in the brain. The 

 hypoglossal nerve does not perforate the brain-case. The amnion 



