8 The Rev. P. Keith o?i the Structure of Aiiimals. 



taste and of sight, but they have no perceptible or indubitable 

 organs of hearing, with the exception of a single tribe, the 

 Crustacea', and the jaws, if any, are lateral. 



The fourth and lowest division of the animal scale includes 

 the zoophytes of preceding naturalists, which Cuvier now de- 

 signates by the appellation of radiate animals, lladiata. They 

 exhibit the greatest simplicity of animal structure, and a pe- 

 culiarity of conformation that cannot be mistaken. In the 

 foregoing divisions, the organs of motion and sensation are 

 found to be symmetrically placed on two respective sides of 

 a certain axis. In the zoophytes [and other Radiata,] the 

 corresponding organs are arranged in rays around a common 

 centre, and hence their appellation of radiate*. The subjects 

 of this division approach the homogeneous character of plants. 

 They have no distinct nervous system, nor specific organs of 

 sense, but merely nervous molecules dispersed throughout a 

 gelatinous mass:}:. In a few of them, you may trace some faint 

 vestiges of a circulation, with respiratory organs on the surface 

 of the body. In the polypi, the intestines are a mere bag 

 without passage; and in the infusory animalcula, the lowest 

 in the animal scale, the individual is merely a homogeneous 

 mass of pulp, endowed with motion and feeling. With these 

 divisions in view, we proceed to exhibit a brief and popular 

 sketch, first, of the external structure, and secondly, of the 

 internal structure, of animals. 



Of the Exteriial Structure of Animals. 



Division I. The Vertebrata. — Of animals of the division 

 Vertebrata, some are viviparous, and furnished with teats by 

 which they are enabled to suckle their young; this is the class 

 of the Mammalia: some are oviparous, and adapted by their 

 structure to the act of flying ; this is the class of Birds : some 

 are destined to live in water, and adapted by their structure 

 to the act of swimming; this is the class of Fishes: and some 

 are doomed merely to crawl upon the earth, and to pass a 

 great part of their existence in a state of stupor; this is the 

 class of Reptiles. 



Class 1. If an individual of the class Mammalia is taken and 

 surveyed at the period of its perfect development, it will be 

 found to be composed of a head, a neck, a trunk, and limbs. 

 We will take our view of these parts as they occur in man, 

 who stands incontrovertibly without a rival at the head of the 



[* It has been shown by Dr. Agassiz, in a paper in our last volume, p. 369, 

 that there is a bilateral symmetry even in certain Radiata^ notwithstand- 

 ing their radiant structure. — Edit.] 



[f Mr. Keith seems here to confound the organization of all the Radiata 

 with that of one of the groups referred to them by Cuvier : in the Starfish 

 and Echini, for example, there is a distinct radiant nervous system. — Edit.] 



