1 80 ECHINODERMA TA. PART in. 



There are pedicellariae scattered among the spines of 

 the sea-urchins which are in constant motion, protruding 1 

 themselves beyond the spines and withdrawing again, 

 snapping their pincers, and grasping firmly anything 

 that comes within their reach, or that is presented to 

 them. The pedicellarise vary much in form and posi- 

 tion in the different genera of the Echinidse ; but they 

 invariably consist of a long, slender, calcareous stem, and 

 generally tripartite head, the whole coated with a gela- 

 tinous fibrous transparent substance. The head of the 

 Pedicellaria globosa is a formidable weapon ; at the apex 

 of each of its three serrated and toothed blades there is 

 a strong sharp spine directed horizontally inwards, so 

 that the three spines cross each other when the blades 

 close, which they do so energetically that nothing could 

 escape from such a grasp. The pedicellarise are curious 

 microscopic objects ; they are extremely irritable, and 

 although their use is unknown, they must be essential 

 to the well-being of the animals, since hundreds are 

 scattered over their shells. 



The spines of the Echinidse vary in shape and structure 

 in the different genera and species. Those of the 

 Scutella form merely a velvety pile. On the common sea 

 urchin the spines are simple, and shed twice in the 

 year ; those on the Amphidetus are both club and spoon - 

 shaped ; and, on the Cidaris, they are large formidable 

 clubs moved by a ball and socket. All the spines, what- 

 ever their form may be, are moved in that manner ; for 

 there are little tubercules on the surface of the shell on 

 which a cup at the bottom of the spines is pressed 

 down by the muscular skin which covers the shell and 

 spines, and by its contractile power it enables the ani- 

 mal to move the spines in any direction. 



The microscopic structure of the calcareous spines is 

 often beautifully symmetrical. Those of the Acrocladia 

 mamillata consist of concentric alternate layers of net- 

 work and sheaths of pillars ; so that a section of the 



