896 ECHINODEE.MA 



manner shown (in side view) at B. The anchor-like appendages 

 project from the surface of the skin, and may be considered as re- 

 presenting the spines of Echinida. Nearly allied to the Synapta is 

 the Chiridota, the integument of which is entirely destitute of 

 ' anchors,' but is furnished with very remarkable wheel-like plates ; 

 those represented in fig. 682 are found in the skin of Chiridoto 

 violacea, a species inhabiting the western pai'ts of the Indian Ocean. 

 These ' wheels ' are objects of singular beauty and delicacy, being 

 especially remarkable for the very minute notching (scarcely to be 

 discerned in the figure without the aid of a magnifying glass) which 

 is traceable round the inner margin of their ' tires/ There can be 

 scarcely any reasonable doubt that almost every member of this cl MSN 

 has some kind of calcareous skeleton disposed in a manner conform- 

 able to the examples now cited ; and it is now generally acknow- 

 ledged that the marked peculiarities by which they are respectively 

 distinguished are most useful in the determination of genera and 

 species. 1 The plates may be obtained separately by the usual 



method of treating the skin 

 with a solution of potass, and 

 they should be mounted in 

 Canada balsam. But their posi- 

 tion in the skin can only be 

 ascertained by making sections 

 of the integument both vertical 

 and parallel to its surface ; and 



,, , these sections, when drv. are 



FIG. 682. Wheel-like plates from skin of 



Cliiriilutit violacea most advantageously mounted 



in the same medium, by which 



their transparence is greatly increased. All the objects of this clas.x 

 are most beautifully displayed by the black-ground illumination, and 

 their solid forms are seen with increased effect under the binocular. 

 The black-ground illumination applied to vert/ thin sections of Echinus 

 spines brings out some effects of marvellous beauty ; and even in these 

 the solid form of the network connecting the pillars is better seen 

 with the binocular than it can be with the ordinary microscope. 2 



Echinoderm Larvae. We have now to notice that most remark- 

 able set of objects furnished to the microscopic inquirer by the larval 

 states of this class ; for our first knowledge of which we were in- 

 debted to the painstaking and widely extended investigations of 

 Professor J. Miiller.-'' All that our limits permit is a notice of two of 

 the most curious forms of these larva- by way of sample of the won- 



1 No systematic account of a species of Holothurian can lie regarded as complete 

 which does not contain an account of the form of its spicules, when these are present. 

 Figures of various forms will he found in Professor Semper's Rr-iseii im Arcliipel tier 

 PhiltppiHcii : HdldtJtnrieii, Dr. Theel's ' CJuiUrngcr ' Reports, and the memoirs of 

 Professors Bell, Lnclwij;, and Selrnku. 



- It may be here pointed out that the reticulated appearance is sometimes de- 

 ceptive, what seems to !>< xiiliil network liein;_; in many instances a liollinv network 

 of passages channelled out in a solid calcareous substance. Between these two con- 

 ditions, in which the relation between the solid framework and the intervening space 

 is completely reversed, there is every intermediate gradation. 



3 Of later works consult especially the ' Selections from Embryological Mono- 

 graphs, ii. Echinodermata,' edited l>y Mr. A. Agassiz, in vol. ix. of the Memoirs oftJic 

 Museum t>f Comparative Zoologr. 



