494 THE MICROSCOPE. 



which they can easily be removed by the application of a 

 gentle force. The concentric markings have in all cases 

 small scalloped edges ; they occur at certain regular inter- 

 vals, and are so many indications of the lines of growth. 

 In the centre there is a circular depression ; and between 

 its circumference and that of the first concentric marking 

 there are eight flattened radii. If the under-surface be 

 examined with a power of 100 linear, the ridges will all 

 be found to have small jointed tubular processes like hairs 

 projecting from them. In no part of this horny tissue is 

 there a trace of a cellular or a reticular structure. 



Wonderfully beautiful as are these creatures in form and 

 colour, the amount of solid matter contained in their 

 tissues is incredibly smalL The greater part of their sub- 

 stance appears to consist of a fluid, differing little, if at 

 all, from the sea-water in which the animal swims ; and 

 when this is drained away, so extreme is the tenuity of 

 the membranes which contained it, that the dried residue 

 of a jelly-fish, weighing two pounds, which was exa- 

 mined by Professor Owen, weighed only thirty grains. 

 The transparency of the tissues render the whole of the 

 Acalephce delightful objects for the microscope. 1 



The Echinodermata belong to the division Annuloida, the 

 most familiar of examples of which are star-fishes and 

 sea-urchins. The labours of that distinguished comparative 

 anatomist and physiologist of Berlin, Johannes Muller, 

 have made us better acquainted with the structure and 

 development of these remarkable animals than with those 

 of most classes of the animal kingdom. The series of feet 

 which protrude along certain fixed lines from the body of 

 an Echinoderm have received the name of " ambulacra ; " 

 and hence, says Mr. Huxley, " we may distinguish their 

 system of vessels as the ambulacral vascular system The 

 existence of an ambulacral vascular system has as yet been 

 demonstrated only in the following orders : — Echinidea, 

 Ophiuridea, Crinoidea, Asteridea, and Holothuridea, with 

 which the fossil Cystidea and Blastoidea are inseparably 

 connected. I therefore limit the Echinodermata to the 



(1) See an excellent paper in the Transactions of the Microscopical Society, 

 " On the Anatomy of Two Species of Naked-eyed Medusae," by G. Busk, Esq. ; 

 also Professor Forbes' works on this fajmly. 



