3 so On the Echinoidea or Sea-Urchins. [Sess. 



d'une maniere approfondie." ^ The calcareous skeleton of the 

 sphaeridia, as well as of the pedicellarise, is beautifully reticu- 

 lated in structure, resembling in its lace-like character the 

 spines of Spatangus purpureus, so well known to most micros- 

 copists. This reticulated structure is peculiai", indeed, to the 

 calcareous parts of the whole group of the Echinodermata, so 

 that even in fossil fragments there is usually little difficulty 

 in determining which belong to this group. 



Before leaving the subject of the test of the sea-urchins, 

 and taking a glance at their internal structure, it may be 

 interesting to note that the calcareous shell is covered, as 

 already said, both on its exterior and interior surface, by a 

 thin and highly sensitive membrane, — the interior lining 

 membrane being also richly ciliated, presumably for the 

 purpose of respiration. Both of these lining membranes 

 have been carefully examined by Prof. Ewart and Dr 

 Eomanes, with the result that an internal nerve plexus 

 or network is found to be spread over their entire surface. 

 Prof. Ewart, who devoted his attention specially to the lining 

 of the interior surface, found, " after a great deal of trouble," 

 that " the internal plexus spreads all over the inside of the 

 shell, and is everywhere in communication with the external 

 plexus by means of fibres which pass between the sides of 

 the hexagonal plates of which the shell of the animal is com- 

 ]30sed." "- By this discovery the remarkable phenomenon is 

 explained of all the external appendages — feet, spines, and 

 pincers — moving in sympathetic unison whenever a stimulus 

 is applied to any one of these organs, seeing they are thus all 

 connected with one common network of nerve fibres. 



The tube-feet have been already noticed when speaking of 

 the minute openings or pores of the ambulacral zones. The feet 

 are used as feelers, as anchors, and as organs of progression ; 

 and they can be stretched out to over-top the longest spines of 

 the animal. The ambulacral system of the whole group of the 

 Echinodermata, which is similar in all its members, is doubly 

 interesting — firstly, because of the ingenuity of its structure ; 

 and secondly, because it is found nowhere else in the animal 



^ ' Etudes sur les Ecliinoidees,' jiar S. Loven, p. 10, 



2 'Jelly-fish, Star -fish, and Sea-urchins,' p. 305 (International Scientific 

 Series, vol. 50). 



