52 THE ANATOMY OF IXYERTEBRATED ANIMALS. 



sisting of a single or double ganglion with two principal lon- 

 gitudinal nerve-cords, is found in many ; and there may be 

 eyes and auditory sacs. 



Upon this foundation a gradual complication of form is 

 based, brought about by — 



1. The elongation of the bilaterally symmetrical body and 

 the formation of a chitinous exoskeleton. 



2. The development of a secondary aperture near the an- 

 terior end of the body, which becomes the permanent mouth. 



3. The division of the mesoblast into successive segments 

 (somites). 



4. The development of two nervous ganglia in each somite. 



5. The outgrowth of a pair of appendages from each so- 

 mite, and their segmentation. 



6. The gradual specialization of the somites into cephalic, 

 thoracic and abdominal groups ; and that of their appendages 

 into sense organs, jaws, locomotive limbs, and respiratory or- 

 gans. 



7. The conversion of the schizoccele into a spacious peri- 

 visceral cavity containing blood ; the reduction of the water- 

 vascular system, and the appearance of pseudo-haemal vessels ; 

 and the replacement of these, in the higher forms, by a heart, 

 arteries, and veins, which contain blood. 



8. The conversion of the simple inner sac of the gastraea 

 into a highly-complex alimentary canal, with special glandu- 

 lar appendages, representing the liver and the kidneys. 



9. A similar differentiation of the genital apparatus. 



10. A gradual complication of the eye, w 7 hich, in its most 

 perfect form, presents a series of crystal-clear conical rods, 

 disposed perpendicularly to the transparent corneal region 

 of the chitinous exoskeleton, and connected by their inner 

 ends with the optic nerves of the prae-cesophageal ganglia. 



By such modifications as these the plan of the simple 

 Turbellarian gradually passes into that of the highest Ar- 

 thropod. 



Starting from the same point, if the mesoblast does not 

 become distinctly segmented * if few, probably not more 

 than three, pairs of ganglia are formed ; if there are no seg- 

 mented appendages, but the chief locomotive organ is a mus- 

 cular foot developed in the neural aspect of the body ; if, in 

 the place of the chitinous exoskeleton, a shell is secreted by 

 a specially modified part of the haemal wall termed the man- 

 tle ; if the schizoccele is converted into a blood-cavity, which 

 communicates with the exterior by an organ of Bojanus, which 



