152 



THE MICROSCOPE AND ITS REVELATIONS. 



this curious body, and especially its most projecting portions, are strength- 

 ened by a frame-work of thread-like calcareous rods (e). In this condi- 

 tion the embryo swims freely through the water, being propelled by the 

 action of the cilia, which clothe the four angels of the pyramid and its 

 projecting arms, and which are sometimes thickly set upon two or four 

 projecting lobes (/); and it has received the designation of pluteus. The 

 mouth is usually surrounded by a sort of proboscis, the angles of which 

 are prolonged into four slender processes (g, g, g, g), shorter than the 

 four outer legs, but furnished with a similar calcareous frame-work. 



545. The first indication of the production of the young Echinus 

 from its ' pluteus/ is given by the formation of a circular disk (Fig. 377, 

 A, c), on one side of the central stomach (#); and this disk soon presents 

 five prominent tubercles (B), which subsequently become elongated into 

 tubular cirrhi. The disk gradually extends itself over the stomach, and 

 between its cirrhi the rudiments of spines are seen to protrude (c); these, 

 with the cirrhi, increase in length, so as to project against the envelope 



FIG. 377. 



Embryonic development of Echinus: A, Pluteuslarva at the time of the first appearance of 

 the disk; a, mouth in che midst of the four-pronged proboscis; 6, stomach; c, Echinoid disk; d, d. 

 d, d, four arms of the pluteus-body ; e, calcareous framework; /, ciliated lobes; g, g, g, g, ciliated 

 processes of the proboscis; B, Disk with the first indication of the cirrhi: c, Disk with the origin 

 of the spines between the cirrhi; D, more advanced disk, with the cirrhi, g, and spines, x, project- 

 ing considerably from the surface. (N.B In B, c, and D, the Pluteus is not represented, its parts 

 having undergone no change, save in becoming relatively smaller.) 



of the pluteus, and to push themselves through it; whilst, at the same 

 time, the original angular appendages of the pluteus diminish in size, the 

 ciliary movement becomes less active, being superseded by the action of 

 the cirrhi and spines, and the mouth of the pluteus closes-up. By the 

 time that the disk has grown over half of the gastric sphere, very little 

 of the pluteus remains, except some of the slender calcareous rods; and 

 the number of cirrhi and spines rapidly increases. The calcareous frame- 

 work of the shell at first consists, like that of the Star-fishes, of a series 

 of isolated networks developed between the cirrhi; and upon these rest 

 the first-formed spines (D). But they gradually become more consoli- 

 dated, and extend themselves over the granular mass, so as to form the 

 series of plates constituting the shell. The mouth of the Echinus (which 



