336 EVEXINGS AT THE MICKOSCOPE. 



Here is in mj cabinet a specimen of a Sea-ni'diin 

 of a less regular form : it is the Heart-Urcliin {A7n- 

 pMdotus cordatui). Essentially, its structure agrees 

 with that of the more globular forms, but it is heart- 

 shaped, and the two orifices, instead of being at oppo- 

 site poles, are separated only by about one third of the 

 circumference. It shows also singular impressed marks 

 on its shell, as if made by a seal on a plastic sub 

 stance. 



But what I chiefly wish to direct your attention to 

 are the spines. Tliese differ much from the kindred 

 organs in Echinus^ being far more numerous, yery 

 slender, curyed, thickening towards the tip, and lying 

 down upon the shell in the manner of hair, whence the 

 species is sometimes called the Hairy Sea-egg. The 

 array of spines has a glittering silky appearance in this 

 dried state. 



"We will now put a few of them under a low power 

 of the microscope, using reflected light and a dark 

 background. They thus present a very beautiful ap- 

 pearance ; elegantly-formed cmwed clubs, made of a 

 substance which seems to be between glass and iyory, 

 haying the whiteness of the latter and the glittering 

 brilliance of the former. The entire surface appears to 

 be exquisitely caryed with excessiyely minute oyal pits, 

 arranged in close-set lines, with the most charming re- 

 gularity. It is the light reflected from the polished 

 bottoms of these pits that imparts to the surface its 

 sparkling brilliancy. At the bottom of the spine there 

 is a little depression, which flts a tiny nipple on a wart- 

 like prominence of the shell, as we saw in Echinxis ; 

 but a little way aboye this point there is a singular 

 projection or shoulder of the calcareous substance. 



