ECHINUS. 123 



§ 1. Echinus sph^era.— Plates XXXI., XXXIL, figs. 1-11. 



WTiile alive this animal is at least five inches and a half in diameter, 

 computing from the opposite extremities of the spines, all the parts being 

 in vigorous, regular, and symmetrical order. 



It may be then circumscribed by about three-fourths of a sphere. 

 Some are more nearly globular, from the greater equality of the vertical 

 and transverse axis, while, from greater inequality, specimens appear dis- 

 torted. But, for the most part, all individuals of the race ai'e surprisingly 

 symmetrical. 



This creature, as is well known, consists of an external shell, con- 

 fining the intestuial parts, wholly invested l^y an infinity of hard, sharp, 

 movealjle spines, resembling those of the terrestrial hedgehog, whence its 

 name is derived. 



Numerous flexible cords or suckers protrude from among the spines, 

 about double their length or more, which extend until reaching some solid 

 substance. The extremity of each then flattens, broadens, and applying 

 firmly, they assist the subject in traversing its-'JDrison. — Plate XXXI. 

 fig. 1, a, a. 



To understand the nature of all this, we niust view the naked shell 

 to find a double set of organs, as above, the spines and suckers. 



A multitude of low spherical segments project from its surface, of all 

 different dimensions. These sustain the basis of the spines, which play 

 on them somewhat as a ball and socket. This part may be therefore 

 viewed as corresponding to the articulation of the bones of other animals. 



Next, the substance of the whole shell is penetrated by a multitude 

 of minute pores, disposed in rows and compartments, for protrusion of the 

 long flexible cords, which are so many suckers, from within. These are 

 properly the instruments of progression. 



Let us suppose a specimen at rest. AU the suckers are retracted 

 within or close on the shell, and the spines motionless. A few of the suckers 

 now protrude ; they search around for a solid foundation, others follow 

 them ; at length many are employed, and the animal is fixed securely in its 

 site. But to change it, those behind are relaxed, A^ilile the contraction 



