LARVAL ECHINODEKMS 



897 



derful phenomena which his researches brought to light, and to which 

 the attention of microscopists who have the opportunity of studying 

 them should be the more assiduously directed, as even the most deli- 

 cate of these organisms have been found capable of such perfect 

 preservation as to admit of being studied, when mounted as pre- 

 parations, even better than when alive. The larval zooids have, by 

 secondary adaptations to their mode of life, acquired a type quite 

 different from that which characterises the adults ; for instead of a 

 radial symmetry they exhibit a bilateral, the two sides being pre- 

 cisely alike, and each having a ciliated fringe along the greater part 

 or the whole of its length. The 

 two fringes are united by a 

 superior and an inferior trans- 

 verse ciliated band, and be- 

 tween these two the mouth of 

 the zb'oid is always situated. 

 The external forms of these 

 larvae, however, vary in a most 

 remarkable degree, owing to the 

 unequal evolution of their dif- 

 ferent parts ; and there is also 

 a considerable diversity in the 

 several orders as to the propor- 

 tion of the fabric of the larva 

 which enters into the compo- 

 sition of the adult form. When 

 the young begins to acquire the 

 characters of the fully developed 

 star-fish and sea-urchin, the 

 parts which are not retained 

 shrivel up, and their substance 

 goes to feed the young form. 



One of the most remarkable 

 forms of Echinoderm larvae is 

 that which has received the 

 name of Bipinnaria (fig. 683), 

 from the symmetrical arrange- 

 ment of its natatory organs. The mouth (a), which opens in the 

 middle of a transverse furrow, leads through an oesophagus, a', to a 

 large stomach, around which the body of a star-fish is developing 

 itself; and on one side of this mouth are observed the intestinal 

 tube and anus (b). On either side of the anterior portion of the 

 body are six or more narrow fin-like appendages, which are fringed 

 with cilia ; and the posterior part of the body is prolonged into 

 a sort of pedicle, bilobed towards its extremity, which also is 

 covered with cilia. The organisation 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 Echinoidea. The temporary mouth of the larva 

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



3 M 



FIG. 683. Bipinnaria asterigera, or larva 

 of star-fish : a, mouth ; a', oesophagus ; 6, 

 intestinal tube and anal orifice ; c, furrow 

 in which the mouth is situated ; d d', bi- 

 lobed peduncle ; 1, 2, 3, 4, 5, 6, 7, ciliated 

 arms. 



