898 ECHINODERMA 



oesophagus of the latter enters on what is to become the dorsal side of 

 its body, and the true mouth is subsequently formed by the thinning 

 away of the integument on its ventral surface. The young star-fish 

 is separated from the Bipinnarian larva by the forcible contractions 

 of the connecting stalk, as soon as the calcareous consolidation of its 

 integument has taken place and its true mouth has been formed, but 

 long before it has attained the adult condition ; and as its ulterior 

 development has not hitherto been observed in any instance, it is not 

 yet known what are the species in which this mode of evolution 

 prevails. The larval zooid continues active for several days after its 

 detachment ; and it is possible, though perhaps scarcely probable, 

 that it may develop another asteroid by a repetition of this process 

 of gemmation. 



In the Bipinnaria, as in other larval zooids of the Aster oidea, 

 there is no internal calcareous framework ; such a framework, how- 

 ever, is found in the larvae of the Echinoidea and Ophiuroidea, of 

 which the form delineated in fig. 684 is an example. The embryo 

 issues from the ovum as soon as it has attained, by repeated ' seg- 

 mentation ' of the yolk, the condition of the ' mulberry-mass,' and 

 the superficial cells of this are covered with cilia by whose agency 

 it swims freely through the water. So rapid are the early processes 

 of development that no more than from twelve to twenty-four 

 hours intervene between fecundation and the emersion of the embryo, 

 the division into two, four, or even eight segments taking place 

 within three hours after impregnation. Within a few hours after 

 its emersion the embryo changes from the spherical into a sub- 

 pyramidal form with a flattened base ; and in the centre of this 

 base is a depression, which gradually deepens, so as to form a mouth 

 that communicates with a cavity in the interior of the body which 

 is surrounded by a portion of the yolk-mass that has returned to the 

 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 quadrangular ; 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. 

 684, A). All parts of this curious body, and especially its most 

 projecting portions, are strengthened by a framework 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 

 (/) ; 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, </), shorter than the four 

 outer legs, but furnished with a similar calcareous framework. 



The first indication of the production of the young Echinus from 

 its ' pluteus ' is given by the formation of a circular disc (fig. 684, 

 A, c) on one side of the central stomach (b) ; and this disc soon 

 presents five prominent tubercles (B), which subsequently become 

 elongated into tubular processes, which will form the 'sucking- 



