LARVAL FORMS OF EC HIN ODERM AT A. 



487 



the painstaking and widely extended investigations of Prof. 

 Miiller. All that our limits permit, is a notice of two of the 

 most curious forms of these larvae, by way of sample of the 

 wonderful phenomena which his researches have brought to 

 light ; so as (it may be hoped) to excite such an interest among 

 those Microscopists in particular who may have the opportunity 

 of pursuing these inquiries, as may induce them to apply them- 

 selves perseveringly to them, and thus to supply the numerous 

 links which are at present wanting in the chain of developmental 

 history. The peculiar feature by which the early history of the 

 Echinoderms generally seems to be distinguished, is this, — that 

 the embryonic mass of cells is converted, not into a larva which 

 subsequently attains the adult form by a process of metamor- 

 phosis, but into a peculiar zooid, which seems to exist for no 

 other purpose than to give origin to the Echinoderm by a kind 

 of internal gemmation, and to carry it to a distance by its active 

 locomotive powers, so as to prevent the spots inhabited by the 

 respective species from being overcrowded by the accumulation 

 of their progeny. The larval zooids are formed upon a type 

 quite different from that which characterizes the adults ; for in- 

 stead of a radial symmetry, they exhibit a hilateral, the two 

 sides being precisely alike, and each having a ciliated fringe 

 along the greater part of the whole of its length. The two 

 fringes are united by a superior and an inferior transverse ciliated 

 band; and between the two, 243 



the mouth of the zooid is al- 

 ways situated. Further, al- 

 though the adult Star-fish and 

 Sand-stars have neither intesti- 

 nal tube nor anal orifice, their 

 larval zooids, like those of other 

 Echinoderms, always possess 

 both. The external forms of 

 these larvse, however, vary in 

 a ifiost remarkable degree, 

 owing to the unequal evolution 

 of their different parts ; and 

 there is also a considerable di- 

 versity in the several orders, 

 as to the proportion of the 

 fabric of the larva which enters 

 into the composition of the 

 adult form. In the fully deve- 

 loped Star-fish and Sea-urchin, 

 the only part retained is a por- 

 tion of the stomach and intes- 

 tine, which is pinched off, so 

 to speak, from that of the Larval zooid. 



321. One of the most remarkable forms of Echinoderm 



Bipinnaria asteti^era, or Larva of Slar- 

 Fish;— a, mouth; a', cesophagus: b, intesti- 

 nal tube and anal orifice ; c, furrow in 

 which the mouth is situated; d d', bilobed 

 peduncle ; 1, 2, 3, 4, 5, 6, 7, ciliated arms. 



