202 LECTURE X. 



hip joint. The spines grow by successive additions, through calcifi- 

 cation of that part of the common organised membranous covering 

 of the shell of the Echinus, which is attached to their base. The 

 varied areolar organisation of the spines affords beautiful microsco- 

 pical objects when viewed in thin transverse slices. 



In the typical genera, Echinus and Cidaris, e. g. the ambulacral 

 pores extend from pole to pole ; but in Scutella and Rotula they are 

 confined to the dorsal aspect of the more flattened spheroid, where 

 they form a five-leaved rosette. 



The tube-feet that issue from the ambulacral pores can be extended 

 beyond the longest spines in the Echinus Sphcera of our own coasts ; 

 they terminate in suckers, which appear to be highly sensitive, and 

 by which the Sea-urchin attaches itself to foreign bodies, and moves 

 along them with a rotatory course, with its mouth downwards, the 

 spines serving to balance and direct the progress of the animal. The 

 bases of the tube-feet communicate with the cavities of the internal 

 vesicles or branchige (Jig. 98, g); their outward surface is ciliated. 

 The terminal sucker of the tube-foot 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 apei'ture conducting to the interior of the 

 ambulacral tube-foot. 



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

 integument, 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 " pedicellaria3," 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 2^ositions 

 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 gemmiform, 

 tridactyle, and snake-headed pedicellai'ias. They are all composed of 

 an internal calcareous axis, and a soft external tissue. 



The gemmiform pedicellariae* are placed around the tubercles, 



* PedicellaricB globifercp, MuUer. 



