ECHINODERMATA. 



151 



FIG. 376. 



narrow fin-like appendages, which are fringed with cilia; and the poste- 

 rior part of the body is prolonged into a sort of pedicle, bilobed towards 

 its extremity, which also is covered with cilia. The organization of this 

 larva seems completed, and its movements through the water become very 

 active, before the mass at its anterior extremity presents anything of the 

 aspect of the Star-fish; in this respect corresponding with the movements 

 of the pluteus of the Echinida ( 545). The temporary mouth of the 

 larva does not remain as the permanent mouth of the Star-fish; for the 

 ossophagus 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 in- 

 tegument on its ventral surface. The 

 young Star-fish is separated from the Bi- 

 pinnarian larva by the forcible contrac- 

 tions of the connecting stalk, as soon as 

 the calcareous consolidation of its integ- 

 ument 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 lar- 

 val zooid continues active for several 

 days after its detachment; and it is pos- 

 sible, though perhaps scarcely probable, 

 that it may develop another Asteroid by 

 a repetition of this process of gemmation. 

 544. In the Bipinnaria, as in other 

 larval zooids of the Asteriada, there is 

 no internal calcareous frame- work; such 

 a frame- work, however, is found in the 



larvae of the EcMnida and Ophiurida, Of Bipinnaria asterigera, or Larva of 



i_ . 1 1 JT a TV oivrv Star-fish: a, mouth; a', oesophagus; o, 



Which the form delineated in Fig. 377 IS intestinal tube and anal orifice ;c, furrow 



an ovaTYiiVlp TVio pmVrvrk iaanpc fvrun T>IP in which the mouth is situated; d d', bi- 



an example. ,ne em Dryo issues iiom tne lobed pedimcle; lf 2 ,3, 4, 5, e, 7, ciliated 

 ovum as soon as it has attained, by repeat- arms, 

 ed 'segmentation' of the yolk ( 581), 



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 seg- 

 ments 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 sur- 

 rounded 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 tri- 

 angular, 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. 377, A). All parts of 



