THE VERTEBRATA. 61 



in front of, and others behind, the oesophagus. A longitudinal 

 vertical section of such an animal, therefore, may be represented 

 by Fig. 30 (II.). 



A similar section of a vertebrated animal shows, on the con- 

 trary, the chief centre of the nervous system not to be perforated 

 by the oesophagus; the latter turning away from it and opening 

 upon the opposite side of the body (Fig. 30, IV.). 



Another structure sharply distinctive of the vertebrate 

 classes is the " chorda dorsal is ' or " notochord," an organ of 

 which no trace has yet been discovered in any of the inverte- 

 brates, though it invariably exists, in early embryonic life 

 at least, in every vertebrate. Before the cerebro-spinal canal is 

 complete, in fact, the substance of the centre of its floor, be- 

 neath the primitive median line of the embryo, becomes differ- 

 entiated into a rod-like cellular structure, which tapers to both 

 its extremities ; and, in a histological sense, remains com- 

 paratively stationary, while the adjacent embryonic tissues are 

 undergoing the most rapid and varied metamorphoses. 



To these great differences between vertebrates and in- 

 vertebrates, in their early condition, many others might be 

 added. In all Vertebrata that part of the wall of the body 

 which lies at the sides of, and immediately behind the mouth, 

 exhibits a series of thickenings parallel with one another and 

 transverse to the axis of the bodv, which mav be five, or more, 

 in number, and are termed the "visceral arches." The inter- 

 spaces between these arches becoming thinner and thinner, are 

 at length perforated by corresponding clefts, which place the 

 cavity of the pharynx in free communication with the exterior. 

 Nothing corresponding with these arches and clefts is known in 

 the Invertebrata. 



A vertebrated animal may be devoid of articulated limbs, 

 and it never possesses more than two pair. These limbs always 

 have an internal skeleton, to which the muscles moving the 

 limbs are attached. Whenever an invertebrated animal possesses 

 articulated limbs, the skeleton to which the muscles are at- 

 tached is external, or is connected w r ith an external body 

 skeleton. 



When an invertebrated animal possesses organs of mastica- 



