14 AMONG THE SEA-URCHINS. 



with which the body is covered, a better purchase, or pushing 

 power, to enable the creature rapidly to disappear beneath the sand ; 

 for the muscular bands, whicli — holding the spines firmly in their 

 position above the glassy nipples — are enabled to move the spoon- 

 shaped spines with a powerful outward stroke, wherewith the sand is 

 shuffled away from the bed. Each spine is covered for some dis- 

 tance above the " ball-and socket " joint with a thin muscular coat 

 full of ligamentous bands. This coating entirely covers the 

 minute spines, to which are attached those curious little append- 

 ages called '"'' Pedicellarice " (PI. I., Figs. 6, 7, 8, 9, and lo), which are 

 fixed upon highly-organised membranous stalks, which move 

 about like the neck of a swan, but are highly contractile (Figs. 6 

 and 10). These organs were originally supposed to be parasites 

 upon the Urchin, but they are now known to act in the way of 

 support to the Echini by seizing hold upon sea-weeds, etc., with their 

 triangular blades or nippers, until the sucker feet of the Echinus have 

 had time to press and attach themselves. When alive, these blades 

 act apparently independently of the will of the animal, for they are 

 constantly opening and closing. Sometimes they are lengthened 

 out and their edges serrated with teeth (Fig. 9), which fit and lock 

 into each other with exquisite precision instantly a thread or leaf of 

 algae touches them. They vary greatly in form, as may be seen 

 in our illustration (Plate I.,. Figs. 7, 8, 9, and 10, and Fig. i, F). 

 Another function they have also been noticed to perform in some 

 species is the removal of excrementitious particles when ejected 

 from the vent. These interesting little organs have been a con- 

 stant source of investigation and puzzle to naturalists for over 

 fifty years. Professor Louis Agassiz originally thought them 

 " infant Echini, which, after their exclusion, affix themselves to the 

 skin of their mother." When denuded of their integument and 

 muscle by boiling in liquor potassae, they form one of the most 

 beautiful microscopic preparations, especially when examined with 

 the polariscope and selenite plate, the texture being remarkable 

 for lightness and porosity, being- formed of a net-work of inter- 

 spaces freely communicating with each other (PI. I., Fig. 8) ; in fact, 

 the whole texture of the Echinus shell is made up of calcified areolar 

 tissue (highly magnified in woodcut. Fig. 2, B), for which every hard 

 portion of the sub-kingdom Echinodermata is so specially noted. 



