ECHINODERMATA. 



Terminology, or descriptive analysis of the component elements of tie test of the 



Echinoidea. 



The test of the Echinoidea is composed of the following parts : 

 a. Five ambulacral areas. 

 h. Five inter-ambulacral areas. 



c. Ten poriferous zones. 



d. An anal opening, and anal membrane and plates. 



e. A mouth opening, and buccal membrane and plates. 



/. Five jaws when organs of mastication exist ; some are edentulous. 



g. Tubercles of various sizes, developed on the outer surface of the plates. 



h. Spines of various forms and dimensions, which are jointed with the tubercles. 



These are the parts essential to be known ; but there are others of secondary import- 

 ance, which will be described hereafter in their proper place. 



The Body of the Echinoidea is divisible into three parts : 



1st. The calcareous envelope or skeleton has a globular, circular, oval, pentagonal, 

 hemispherical, conoidal, or discoidal form ; it is composed of a framework of pentagonal, 

 hexagonal, and polygonal calcareous plates. This testaceous box is called the test. It 

 is the form, the test of Agassiz ; the general form, the test of Desmoulins ; la coquille 

 of d'Orbigny. 



2d. The visceral cavity, which contains the organs of digestion, respiration, circulation, 

 and generation, is formed entirely by the calcareous skeleton. 



3d. The external surface of the test is covered with spines, which are moveably articu- 

 lated with the tubercles developed on the surface. 



The normal position of the body. — In describing the different parts of the test of the 

 Echinoidea, it is assumed that the urchin, the common purple-heart urchin, Spatangus 

 purpureus, Muller, for example, is placed before the observer ; or, the common chalk urchin, 

 Micraster cor-anguinum, Klein, will answer equally well. The side with the single ambula- 

 crum lodged in the sulcus, and the mouth in that third of the base, is the anterior region. 

 The four other ambulacra are disposed in pairs, and correspond to the right and left sides 

 of the observer's body ; there is, therefore, a right antero-lateral and a right postero- 

 lateral ; a left antero-lateral and left postero-lateral ambulacral area. The side having the 

 single inter-ambulacrum in the middle, and the anal opening in the upper part of the 

 border, is the posterior region. The four other inter- ambulacra are likewise disposed 

 in pairs, two of which, with the single ambulacrum, form the anterior part of the test ; 

 the other pair, with the pairs of ambulacra, forming the sides, and the single inter- 

 ambulacrum its posterior part. 



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