508 THE MICROSCOPE. 



fully complicated design, executed on a groundwork broad 

 as time, and whose scope and bearing are deep as 

 eternity." 1 



The Synaptidce are characterised by a total absence of 

 ambulacra, the motions of the animals being assisted by 

 peculiar anchor-like processes which project from the skin, 

 and roughen the surface of the animal. The spiculum re- 

 presented in fig. 239 is serrated on the convex edge, and, 

 Dr. Herepath says, apparently belongs to S. Galliermii, 

 but that the drawing of the animal near it is singularly 

 inaccurate, although taken from Professor Forbes' work ; 

 and that the oral tentacles are imaginary developments of 

 S. digitata. See Herepath, " On the Genus Synapta," 

 Quar. Journ. Micros. Science, 1865, p. 1. The spicules 

 are beautiful objects for polarised light. (Plate IV. No. 

 87, shows a side view of one set in the skin.) 



Holothuridea, " Sea-cucumbers." — In this family the 

 body acquires a slug-like form. The radiate structure is 

 in fact scarcely recognisable in these animals, except in 

 the arrangement of the tentacles which surround the 

 mouth. The body is always more or less elongated, with 

 the mouth at one end and the anal opening at the other ; 

 the calcareous deposit in the skin is reduced to scattered 

 granules ; and in one family the ambulacra are entirely 

 wanting. 



The integument consists of a number of minute reticu- 

 lated plates, set closely on the substance of the skin. The 

 forms of the plates are various, as well as the spicula set 

 in them. The Australian seas furnish many varieties : 

 Plate VIII. Xos. 171 and 172, are representations of 

 plates and spicula under polarised light. Objects of this 

 class are also well suited for black-ground illumination. 



The structure of the spines and other solid parts of the 

 skeleton of Echinodermata can only be displayed by 

 making thin sections, in the way described for cutting 

 bone, at page 209. Their peculiar texture requires that 

 certain precautions should be taken to prevent the section 

 from breaking whilst being reduced to a desirable thin- 

 ness, and to prevent the interspaces of the network from 

 being clogged by the particles abraded in the reducing 



(1) Miller's Testimony of tHe Rocks. 



