136 ECHJNODERMATA. 



without appearing to form any portion of that series of devclope- 

 ment which we have hitherto been able to trace. 



But this apparent want of conformity to the general laws of 

 developeinent vanishes on more attentive examination ; so that we 

 may not only trace the steps by which every family of this exten- 

 sive class merges insensibly into another, but perceive that, at the 

 two opposite points of the circle, the ECHINODERMATA are inti- 

 mately in relation with the POLYPS on one hand, while on the 

 other they as obviously approximate the annulose animals, to which 

 the most perfectly organized amongst them bear a striking resem- 

 blance. 



It would be impossible within our present limits to do more 

 than lay before the reader the most important types of structure 

 which the Echinodermata exhibit ; it must, nevertheless, be under- 

 stood that innumerable intermediate families connect the different 

 genera ; so that, however dissimilar the examples we have selected 

 for the purpose of exhibiting their general habits and economy 

 may appear, the gradation which leads from one to another is 

 easily traced. 



(180.) Crinoidce. We have already found that many tribes of 

 polyps secrete calcareous matter in large quantities, so as to con- 

 struct the solid skeletons or polyparies, which generally seem to 

 be placed external to their soft and irritable bodies, but occasion- 

 ally, as in Pennatula, within the living substance. Let us for a 

 moment suppose a polyp supported upon a prolonged stem, and 

 that, instead 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 animal with solid pieces, of definite 

 form ; such pieces being connected 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 di- 

 rection. A polyp so constituted would obviously, when dried, 

 present an appearance similar to what is depicted in the annexed 

 engraving (Jig- 57), representing an Encrinoid Echinoderm in 

 its perfect condition. That animals thus allied to polyps in 

 their outward form have in former times existed in great num- 

 bers upon the surface of our planet is abundantly testified by the 

 immense quantities of their remains which are met with in various 

 calcareous strata, but their occurrence in a living state is at 



