40 



COMPA RA TIVE A NA TOM Y 



chitinous or calcareous spines, setse, scales, granules, etc., varying in number and 



arrangement according to the genus and species. 



Each spine, as a rule, ari.ses as a globular vesicle within an epithelial papilla and 



above a very large formative cell (Fig- 



50). As it grows, it is pushed upwards by 



the newly - forming cuticular layers. The 



formative cell at its base persists, but remains 



connected with the epithelial papilla only liy 



a pi-otoplasmic process which continually 



lengthens, and may surround itself with a 



nucleated sheath. In fully-develojied spines, 



the remains of this cell are still found as a 



small terminal swelling (Endkolbchen). 



There are, however, spines and specially 



flat scale- or plate-like calcareous formations 



in the integument which do not arise from 



single large foimative cells, but are probably 



produced by several cells in the base of an 



epithelial papilla. 



Just as we have recognised the tegmentum 

 Fig. 50.— a, B, C, Three stages in de- -4.1 i- 1 4. j. 1 1 



. , • ■ 41. ^J^4 , 4.4 covering the articuiamenta to be merely a 

 velopment of a spme in the Chiton (after ° •' 



Blumrioh), .liagrammatic. st, Spine ; Iz, its special portion of the general cuticle, so we 

 formative cell ; e, epithelium ; c, thick may further recognise in the articuiamenta 

 cuticle secreted by the epithelium; eV, the homologues of the calcareous spines, 

 terminal swelling (Endkolbchen) = remains ^^^^ ^^.j^j^j^ ^^^ developed in the integu- 



of the formative cell. ^ , , mi - 1 



ment of the mantle. ihe articuiamenta 



would thus be nothing more than very large and expanded calcareous scales. 



t 15 



Fio. 51.— Transverse section through a Chiton near the nephridlal apertures, higlily 

 diagrammatic (after Sedgwick), somewhat modified. 1, Pericardium; '2, ventricle; 3, auricle; 

 4, branchial "vein"; 5, branchial groove (mantle cavity); (!, gill (ctenidium) ; 7, foot; 8, pleuro- 

 visceral connective; !>, branchial "artery"; 10, secondary coelom ; 11, intestine; 12, posterior 

 portion of the gonad lying below the pericardium ; 13, 14, the two posterior branches of the 

 nephridium, one of which (13) opens into the branchial groove (at IG), the other (14) being connected 

 in a way not here depicted with the pericardium ; 15, pedal nerves. 



This view, finally, leads to the conclusion that the shell (if it may here be so 



