190 



THE SCIENCE OF LIFE. 



monly known as Sea-slugs, Sea-cucumbers, or Trepangs. 



The body is elongated and soft, with a tough contractile 



skin containing calca- 

 reous spicules. One 

 end, the head, has a 

 simple aperture for 

 a mouth, encircled 

 with feathery tenta 

 cles. In the Holo- 

 thurice proper, loco- 

 motion is effected by 

 rows of ambulacral 

 tube-feet, but in the 

 Synaptidce there are 

 no ambulacra, and 

 the animal moves 

 9 by means of anchor- 

 shaped spicuta which 

 are scattered in the 

 integument. Animals 

 of this order have 



FIG. 91. Embryonic development of Echinus: the singular DOWCr of 

 A. Pluteus larva at the time of the first appearance 



of the disk. a. Mouth in the midst of the four-pronged ejecting all their in~ 

 proboscis. . Stomach, c. Echinoid disk, dddd.fovit 



arms of the Pluteus body. e. Calcareous frame-work, temal Organs, SUTviv*- 

 ./". Ciliated lobes, g g g g. Ciliated processes of the . , i / i 

 proboscis. B. Disk, with the first indication of the ln g tne 1OSS Ol tnCSC 

 cirrhi. C. Disk, with the origin of the spines between ~ oH -c otirl aff/^rro,-r1 

 the cirrhi. D. More advanced disk, with the cirrhi P artS > a!K 

 and spines projecting considerably from the surface. t*pr>T"OflllcinP" I'll PIT! 

 (N.B. In figs. B, C, and D, the Pluteus is not repre- 

 sented, its parts having undergone no change, save in Their Vermiform lar- 

 becoming relatively smaller.) 



va has no skeleton. 



6. The Radiate type of animal life well illustrates the 

 intellectual plan, or typical design, of living forms, and 



