572 DEVELOPMENT OF ECHINUS FROM PLUTEUS-LARVA. 



liquid granular state. Subsequently a short Intestinal tube is 

 found, with an Anal orifice opening on one side of the body. The 

 pyramid is at first triangular, but it afterwards becomes quad- 

 rangular ; and the angles are greatly prolonged round the mouth 

 (or base), whilst the apex of the pyramid is sometimes much 

 extended in the opposite direction, but is sometimes rounded-off 

 into a kind of dome (Fig. 297, a). All parts of this curious body, 

 and especially its most projecting portions, are strengthened by a 

 frame-work of thread-like Calcareous rods (e). In this condition 

 the Embryo swims freely through the water, being propelled by 

 the action of the Cilia, which clothe the four angles of the pyramid 

 and its projecting arms, and which are sometimes thickly set upon 

 two or four projecting lobes (J) ; and it has received the designa- 

 tion of Pluteus. The Mouth is usually surrounded by a sort of 

 Proboscis, the angles of which are prolonged into four slender pro- 

 cesses (g, g, g, g), shorter than the four outer legs, but furnished 

 with a similar Calcareous frame- work. 



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

 from its 'Pluteus,' is given by the formation of a circular Disk 

 (Fig. 297, a, c), on one side of the central Stomach (b) ; and this 

 Disk soon presents five prominent tubercles (b), which subsequently 

 become elongated into tubular Cirrbi. 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 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 Ciliary movement becomes less active, being superseded by the 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 is altogether distinct from that of the Pluteus) is 

 formed at that side of the granular mass over which the Shell is 

 last extended ; and the first indication of it consists in the appear- 

 ance of the five calcareous concretions, which are the summits of the 

 five portions of the frame-work of Jaws and Teeth that surround it. 

 All traces of the original Pluteus are now lost ; and the Larva, 

 which now presents the general aspect of an Echinoid animal, 

 gradually augments in size, multiplies the number of its Plates, 

 Cirrhi, and Spines, evolves itself into its particular Generic and 

 Specific type, and undergoes various changes of internal structure, 

 tending to the development of the complete organism.— In collect- 



