ECHINODERMATA. 163 



to be effected ; and that without ever deranging the proportions of 

 the whole fabric, or necessitating a loosening of its parts ? No 

 other contrivance could apparently have been adequate to the pur- 

 pose : nevertheless, by the structure adopted, we see how admirably 

 the growth of Echinus proceeds in all directions; for the living and 

 vascular membrane which covers the whole external surface of the 

 body dips down between the edges of the various calcareous pieces, 

 and continually deposits around the margin of each, successive 

 layers of earthy particles, which, assuming a semi-crystalline ar- 

 rangement, progressively increase the dimensions of each individual 

 plate. But the continual augmentation in size, which is thus 

 going on, is attended with no change in the mathematical figure of 

 any given piece of the skeleton ; so that, as they all increase in 

 diameter by the unceasing deposition of earthy matter around the 

 circumference of every plate, the spherical shell gradually expands, 

 without in any degree altering its form or relative proportions, 

 until it has acquired the mature dimensions belonging to its 

 species. 



(204.) The tubular suckers or retractile feet, which are pro- 

 truded at the pleasure of the animal from the countless minute 

 apertures seen in the ten rows of ambulacral plates, are so similar 

 in all essential points to those of Asterias already described, 

 that little further need be said concerning their structure, or the 

 mechanism by which their motions are effected. The tubular part 

 of each foot communicates with the interior of the shell by two 

 branches which pass through two apertures, and these branches in 

 some species (as Echinus saxatilis) receive offsets from the ves- 

 sels which run along the centre of each ambulacral groove, and 

 convey to the feet the fluid by which their distension is effected. 

 In Echinus esculentus the feet open into a plexus of vessels, 

 formed in leaf-like membranes, equal in number with the feet, and 

 disposed in double rows upon the inner surface of the ambulacral 

 pieces,* by the intervention of which they are connected with the 

 canals above mentioned. 



(205.) The tubercles upon the external surface of the shell of 

 the Echini support a corresponding number of long spines, which, 

 as well as the apparatus of suckers, are employed as locomotive 

 agents. These spines vary materially in their form and propor- 

 tionate size, and even in their internal structure and mode of 



* Cyclopaedia of Anat. and Phys. art. ECHINODERMATA. 



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