40 



COMPARATIVE ANATOMY 



CHAP. 



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



arrangement according to the genus and species. 



Each spine, as a rule, arises 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 by 

 a protoplasmic process which continually 

 lengthens, and may surround itself with a 

 nucleated sheath. In fully-developed 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 formative cells, but are probably 

 produced by several cells in the base of an 

 epithelial papilla. 



Just as we have recognised the tegmentum 

 covering the articulamenta to be merely a 



FIG. 50. A, B, C, Three stages in de 

 velopment of a spine in the Chiton (after 



Blumrich), diagrammatic, st, Spine ; bz, its special portion of the general cuticle, so we 

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

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

 terminal swelling (Endkolbchen) = remains 

 of the formative cell. 



ment of the mantle. The articulamenta 



scales, etc., which are developed in the integu- 

 ment of the mantle. The articu 

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



y* ^ 



FIG. 51. Transverse section through a Chiton near the nephridial 'apertures, highly 

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

 4, branchial "vein"; 5, branchial groove (mantle cavity); 0, gill (ctenidium) ; 7, foot; 8, pleuro- 

 visceral connective; 9, 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 16), 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 



