120 LECTURE X. 



tubes comnmnicate with the cavities of the internal vesicles or 

 branchiae. The terminal sucker of the tube is supported by a circle 

 of five or, sometimes, four reticulate calcareous plates, which intercept 

 a central foramen, and by a single delicate reticulated perforate 

 plate on the proximal side of the preceding group. The centre 

 of the suctorial disc is perforated by an aperture conducting to the 

 interior of the ambulacral tube. 



I have reserved the notice of another class of appendages to the in- 

 tegument, not only of the Echini^ but of the Asterise, for this part of my 

 discourse, because they are most developed, most varied in structure, 

 and have been most minutely investigated in the species of the 

 globular family of Echinoderms. The appendages to which I allude 

 are called " Pedicellariae," and consist of a dilated end or head, 

 usually prehensile, supported by a slender stem or pedicel. They 

 present different forms, which hold constant and determinate positions 

 in the crust of the Echinus : they seem at no season to be absent, and 

 must therefore form part of the integral organisation of the Echino- 

 derm. They have however been conjectured by some naturalists to be 

 parasitic animals ; by others to be the young of the Echini, to which 

 they are attached. 



In the Ech. lividus, Professor Valentin, to whom we owe the 

 most minute descriptions of these bodies, divides them into gemmi- 

 form, tridactyle, and snake-headed pedicellarise. They are all com- 

 posed of an internal calcareous axis, and a soft external tissue. 



The gemmiform pedicellarise * are placed around the tubercles, 

 especially the largest ones ; their pedicel is long and slender ; their 

 capital resembles the bud of a flower, defended by three sepals, the 

 apex of each of which is produced inwards in the form of two pairs 

 of long and slender teeth. The quadridentate sepaloid plates can be 

 divaricated and approximated, and constitute a very effective pre- 

 hensile instrument : they are highly irritable; a needle introduced 

 into their grasp is instantly seized. The ciliated gemmule of any 

 parasitic coralline, which might settle about the base of a spine, and 

 there commence its growth, would be liable to be seized and uprooted 

 by the prehensile gemmiform pedicellariae, which are of microscopic 

 minuteness. 



The tridactyle pedicellarise are of larger size, are visible to the naked 

 eye, and fit to grapple with and dislodge young sedentary parasites of 

 larger species, as Cirripeds and Conchifers. They are found more 

 particularly around the large tubercles of the interambulacral plates 

 which support the largest spines. Their capital is longer, narrower, 



* Pedkellaria glohifera Muller. 



