52 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



sisting of a single or double gangliou 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 schizocoele 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 gastra^a 

 into a hig-hly-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, which, 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 pr^-oesophageal 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 i 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 bod}^; 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 schizocoele is converted into a blood-cavity, which 

 communicates with the exterior by an organ of Bojanus, which 



