162 



ECHINODERMATA. 



Fig. 80. 



exhibited by the ECHINODERMATA ; it must nevertheless be understood 

 that innumerable intermediate families connect the different genera ; so 

 that, however dissimilar the examples we have selected for the purpose 

 of elucidating their general habits and economy may appear, the grada- 

 tions leading from one to another are easily discoverable. 



(433.) CRINOID^:. We have already found that many tribes of Polyps 

 secrete calcareous matter in large quantities, constructing for themselves 

 the solid skeletons or polyparies which generally seem to be placed 

 external to their soft and irri- 

 table bodies, but occasionally, as 

 in Pennatula, within the living 

 substance. Let us for a moment 

 suppose a polyp supported upon 

 a prolonged stem, and that, in- 

 stead of depositing the earthy 

 particles externally, they should 

 be lodged in the substance of the 

 polyp itself, so as to fill the 

 pedicle, the body, the tentacula 

 around the mouth, and all the 

 appendages belonging to the ani- 

 mal, with solid pieces, of definite 

 form ; such pieces, being con- 

 nected together by the soft parts, 

 and surrounded on all sides with 

 irritable matter, would thus form 

 a complete internal skeleton, 

 giving strength and support to 

 the entire animal, and at the 

 same time allowing flexure in 

 every direction. A polyp so con- 

 stituted would obviously, when 

 dried, present an appearance 

 similar to what is depicted in the annexed engraving (fig. 80), repre- 

 senting an Encrinoid Echinoderm in its perfect condition. That ani- 

 mals thus allied to polyps in their outward form have, in former times, 

 existed in great numbers upon the surface of our planet is abundantly 

 testified by the immense quantities of their remains met with in various 

 calcareous strata ; but their occurrence in a living state is at present 

 extremely rare : one minute species only has been detected in our own 

 seas*; while specimens of larger growth, such as that represented in 

 the engraving, derived from tropical climates, are so seldom met with, 

 that it is fortunate one or two examples have been found, to reveal 



* Thompson (J. W.), Memoir concerning the Pentacrinus europceus. Cork, 

 1827, 4to. 



Encrinus. 



