502 THE MICROSCOPE. 



ambulacral nerves pass to reach it. In the Aster idea, the 

 circle lies at the extreme limit of the soft membrane, which 

 surrounds the mouth, and may be readily exposed by cut- 

 ting away the hard inter-ambulacral oral lips. In the 

 Holothuridea it lies immediately beneath the perisoma of 

 the oral disc. The only known organs of sense in the 

 Echinodermata are the pigmented " eye-spots," developed 

 in connexion with the ends of the ambulacral nerves, and 

 on the oral nervous circle in many Holothuridea. 



The great majority of the Echinodermata commence 

 their existence as free-swimming larvse covered with cilia, 

 but a great difference exists in their further course, accord- 

 ing as they belong to the Asteridea, the Holothuridea, and 

 the Crinoidea on the one hand, or to the Echinidea and 

 Ophiuridea on the other. Of the development of the 

 Crinoidea we know very little, beyond the observations of 

 Mr. Thompson, that the larva of the Comatida is provided 

 with several transverse bands of cilia, almost like that of 

 a Holothuria, and that the development of the Echinoderm 

 commences while the larva is still free. At a later period, 

 the young Comatida is seated upon a long, jointed stem, so 

 as to resemble a Pentacrinus ; and it becomes detached 

 from this stem, in assuming its adult condition. 



Mr. Huxley, after mature examination of this class of 

 animals, says, " he can see no reason for retaining them 

 amongst the Radiata of Cuvier, but, on the other hand, 

 thinks them properly placed among the Annuloida." 



The skeleton of the Echinoderms generally consists of an 

 assemblage of plates, or joints, of calcareous matter. The 

 minute structure of which presents a reticulated character, 

 and the solid parts are usually composed of a series of 

 super-imposed laminae or scales. The openings, or areolae, 

 in one layer being always placed over the solid cell-walls 

 of the layer beneath it, the spines are situated on the ex- 

 ternal surface of the shell ; they are generally of a conical 

 figure, and are articulated with the tubercles by a ball- 

 and-socket joint. When a thin transverse section of one of 

 these spines is examined with the naked eye, it appears to 

 be made up of a series of concentric layers, varying 

 considerably in number; not, however, with the size of 

 the spine, but with the distance from the base at which. 



