128 ECHINODERMATA. 



pure wliite^ and the epidermis criinwuu red, or a .stellate figure arises 

 from the contrast ; so that we seem to contemplate entirely a different 

 animal. Also the suckers are white, grey, or reddish. But they are 

 commonly faint, or nearly colourless. The whole animal and its parts 

 are likewise found pure white, but nothing more than mere varieties 

 seem to be thus indicated. The animal usualty darkens with age. 



The bare shell exposes five larger and five smaller longitudinal com- 

 partments, studded with tubercles of various dimensions, whereon the 

 spines were borne. Interspersed between these compartments, which all 

 broaden in the middle, and contract towards the ends, exactly like the di- 

 visions of an orange, are interposed numerous lines or stripes of pores, for 

 transmission of the suckers. Thus a portion is solid, and a portion porous. 



Now the orifice for the dental apparatus is seen below, and that 

 which opens above at the vertex of the shell becomes alike conspicuous. 

 Both are pentangular, the former being about an inch wide, and the 

 latter nl:)out half an inch, in a shell of three inches diameter. 



The thickness of such a shell is about the sixteenth of an inch. 



Like the planked hull of a vessel, the sohd portions are wholly built 

 up as it were of smaller pieces, of peculiar form, their size regulated by 

 the dimensions of the shell. Tlie length of each is about twice its breadth ; 

 three sides are rectilinear, the fourth angular. These are compartments 

 rather in appearance than reality. The principal large compartments are 

 composed of several smaller compartments. 



It is extremely difficult to understand what takes place among the 

 parts as the shell is enlarged. All undergo such a change as is propor- 

 tioned to the dimensions of the specimen. The pieces of the compart- 

 ments forming the solid portion are much extended in length and breadth ; 

 the size of the dental apparatus increases ; the pores in the substance of 

 the shell, and the upper and lower orifices, are all greatly widened. 



These changes are seen best by comparing several Echini, large and 

 small. 



Both the higher and the lower orifices form pentagonal external de- 

 pressions in the surface of the shell. The latter is chiefly connected 

 with the mouth. Above, the upper orifice is surrounded internally by 

 the ovarium, which sometimes rises in five clumsy conical portions from 



