132 Dr Barry on the Unity of Structure 



It is obvious from the above, 



Istly, That in theOsteozoa, the central portion of the Nervous 

 System ; in the Arthrozoa, the organs of locomotion, mandibles, 

 &c., are the especial objects, in the early stages of develop- 

 ment. 



2dly, That the central part of the Aniwial layer is appropriated 

 accordingly. Thus it may, perhaps, be said, that parts corre- 

 sponding to the Lamina? dorsales of the Osteozoa, go to form the 

 Extremities chiefly, in the Arthrozoa. 



3dly, That the upper tube in the Arthrozoa is imperfect, 

 though there is evidently a tendency in the extremities to its 

 formation.* 



4thly, That, from the direction taken by their extremities^ 

 the Arthrozoa must move about, with the thorax and abdomen up- 

 permost; the relative position of the Fundamental organs being 

 reversed. The organs also, formed out of the Mucous and 

 Vascular layers, are found to be inverted, if compared with 

 corresponding parts in the Osteozoa ; but there occurs such an 

 adjustment in the situation of the external parts, as, for example, 

 in that of the mouth and organs of sense ; and, as what in the 

 Osteozoa is the extensor, becomes in the Arthrozoa the flexor 

 side of the body ; that, so far as these are concerned, it cannot 

 be said that the Arthrozoa move about on their backs. Bather 

 may it be affirmed, with Valentin, that " they have no true 

 back, but only the tendency to form one."-|- But their thorax 

 and abdomen are certainly inverted. 



5thly, That the situation of what these animals have of the 

 central portion of a Nervous System, is a part of the body cor- 

 responding very nearly to that occupied by the central portion 

 of the Nervous System in the Osteozoa ; viz., it is in the former 

 (Arthrozoa) situated in the under in the latter (Osteozoa) in 

 the upper part of the animal layer, supposing each of these 

 Classes of animals to be situated above the yolk. 



6thly, That the term " dorsal" vessel, is calculated to mislead ; 



themselves, in no small degree, ventral as well as locomotive properties, ex- 

 hibit formal approximations to both Gastrozoa and Arthrozoa in their deve- 

 lopment. (Burmeister, 1. c. p. 419.) 



* Such is the idea of Valentin, 1. c. p. 608. 



^ JLoco citato, p. 607. 



