168 Description of the Plates. 



to Invertebrata which are 'Neuropods/ having their main nerve 

 centre placed in the same relative position as that occupied by the 

 heart in the former class''. 



Above the heart we see the digestive canal, and above the 

 digestive canal the craniospinal, and the nervous centres it 

 contains. But the great vessel, known as the aorta (^;), which 

 carries blood from the heart to the organs of the body generally, 

 interposes itself in all Vertebrata between the lower surface of the 

 craniospinal and the upper surface of the digestive tube. 



A minor point distinctive of Vertebrata is the presence of a 

 spleen. 



The peculiarities of the tegumentary system are distinctively 

 mammalian, as are also the following points in the anatomy of 

 the internal organs shown in this figure : the suspension of the 

 lungs freely in closed cavities, the so-called ' pleural ' cavities ; 

 the presence of a perfect diaphragm {c) supplied by a phrenic 

 nerve originating in the region of the neck; the smoothness of 

 the external surface of the kidney ; the persistence of the aortic 

 arch of the left side {p) ; and the presence of an omentum or 

 epiploon [w). 



The scalpriform incisors characteristic of the order Rodentia 

 are concealed in this profile view by the lips, but the figure shows 

 well the great size of the masseter muscle, by the contraction of 

 which the characteristic anteroposterior movement of the lower 

 upon the upper jaw is effected. The great size of the organs of 

 special sense relatively to the entire bulk of the animal, and of 

 the hind relatively to the fore limbs, are characteristic, though not 

 universally nor exclusively, of Rodents. 



^ These two latter names are more strictly accurate than the two former, for in 

 some Invertebrata the main nerve-centres are separated from the viscera of organic 

 life by more or less perfect septa, formed by separately calcified ossicles in the Aste- 

 roidea, or by the prolongation inwards of processes from the external skeleton, as in 

 many Crustacea and Insecta. But in all such cases the neural stUl remains the 

 motor surface, and the term ' Neuropod ' applies to them as strictly as it does to any 

 of the Vermes or Mollusca. For the ossicles of Asteroidea, see pi. x., infra ; and 

 for the ' endophragmal' skeleton, in Crustacea, see Huxley, 3Iedical Times and 

 Gazette, Feb. 21, 1859, p. 181 ; Nov. 7, 1857, p. 467: in Insecta, see Burme-'ster, 

 Manual of Entomology, translated by W. E, Shuckard, p. 236; and for figures of 

 it, see Spence Bate, British Association Report for 1855, pi. xxii., figs, i and 2 : 

 Straus Durckheim, Considerations generalea sur I'Anatomie Comparee des Animaux 

 Articul^s, pl. ii., fig. i . 



