192 ECHINODEEMATA. 



integument whereby it is secreted, the interior of the shell being filled 

 with sea- water, in which the viscera are loosely suspended. But a 

 second and more important reason for the employment of so many pieces 

 in the construction of the shell of an Echinus is to be derived from 

 examining the mode in which the animal grows. Were it to retain the 

 same dimensions throughout the whole period of its life, or could it, at 

 stated intervals, cast off its old investment and secrete a new and more 

 capacious covering as growth rendered the change necessary, a simple 

 earthy crust would have been sufficient, without the presence of such an 

 immense number of sutures and joinings. The calcareous plates of the 

 Echinus, it must be remembered, are merely secreted from the soft 

 parts, having no vital action going on within them whereby, as in the 

 bones forming the skeletons of vertebrate animals, a continual depo- 

 sition of fresh particles could be effected, allowing of extension by inter- 

 stitial deposit. How, therefore, could the growth of the Echinus be 

 provided for ? How is the gradual expansion of the entire shell, thus 

 composed of a dense and extravascular crust, to be effected and that 

 without ever deranging the proportions of the whole fabric, or necessi- 

 tating a loosening of its parts ? No other contrivance could apparently 

 have been adequate to the purpose : nevertheless we see how admi- 

 rably, by the structure adopted, the growth of these creatures proceeds 

 in all directions ; for the living and vascular membrane that 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 arrangement, 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 still 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. 



(506.) The tubular suckers or retractile feet, that are protruded at 

 the pleasure of the animal from the countless minute apertures seen in 

 the ten rows of ambulacra! 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 whereby their motions 

 are effected. The tubular part of each foot communicates with the in- 

 terior of the shell by two branches passing through two apertures ; and 

 these branches, in some species (as Echinus saxatilis), receive offsets 

 from the vessels that run along the centre of each ambulacral groove, 

 and convey to the feet the fluid by which their distention 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 



