ZOOLOGY. 547 



3316. In Birds therefore the vitelline canal is the 

 ccecuni also, and the two cceca, falsely so called, which 

 are placed upon the side of the rectum cannot represent 

 the coecum coli, or else the Bird must have three of these 

 blind appendages or sacs. 



3317. The spleen here appears for the first time; 

 while the pancreas divides into a number of ccecal appen- 

 dages. 



Thorax. 



3318. The thorax of the Fish is reduced within very 

 narrow limits ; it is like the first thoracic formation, and 

 is thus a Mussel's thorax. The branchiae with their 

 opercula are similarly formed to the branchial plates 

 and shells of the Bivalve Mollusca. The thorax is there- 

 fore attached only externally to the body, and the Fish is 

 to be viewed as a Mussel, from between whose shells a 

 monstrous abdomen has grown out. 



3319. But this Molluscan thorax is conjoined with 

 animal systems, and has assumed their noble or elevated 

 type of structure. Here therefore the osseous and sar- 

 cose system blend together, and the higher formation of 

 the thorax emerges into view. 



3320. For the first time an advance is made towards 

 the formation of a trachea, namely, by the branchial 

 framework or skeleton, which opens into the mouth, 

 and corresponds therefore properly to the larynx. Fishes 

 are therefore the first animals which breathe through the 

 mouth. In all the preceding classes the air entered the 

 body, or the water found access to the branchiae, by other 

 routes. 



3321. They may be termed Mouth-breathing animals, 

 for the first formation of the trachea extends no further 

 than to its junction or communication with the mouth ; 

 since, for it to be continued into the head and open self- 

 substantially as a nasal organ, constitutes a second step 

 in advance, which cannot be ventured upon in an Ab- 

 dominal animal. In Fishes everything relates to the 

 abdomen, and this is expressed by the first union of the 



