54 ON THE ORIGIN AND [CHAP. 



transferred back to a period when it had not to 

 provide for itself, the Nauplius has become degraded 

 into a mere skin ; in Ligia this larva-skin has lost 

 the traces of limbs, and in Philoscia it is scarcely 

 demonstrable." 



The Echinodermata in most cases " go through a 

 very well-marked metamorphosis, which often has 

 more than one larval stage. . . . The mass of more 

 or less differentiated sarcode, of which the larva, or 

 pseud-embryo, as opposed to the Echinoderm within 

 it, is made up, always carries upon its exterior certain 

 bilaterally-arranged ciliated bands, by the action of 

 which the whole organism is moved from place to 

 place ; and it may be strengthened by the super- 

 addition to it of a framework of calcareous rods." a 

 Miiller considered that the mouth and pharynx of 

 the larva were either absorbed or cast off with the 

 calcareous rods, but were never converted into the 

 corresponding organs of the perfect Echinoderm. 

 According to A. Agassiz, however, this is not the 

 case, but on the contrary " the whole larva and all 

 its appendages are gradually drawn into the body, 

 and appropriated." 2 



Fig. 39 represents the larva of a sea-egg (Echino- 

 cidaris) after Miiller. 3 The body is transparent, 

 shaped somewhat like a double easel, but with two 

 long horns in front, which, as well as the posterior 



1 Rolleston, " Forms of Animal Life," p. 146. 



2 A. Agassiz, "Embryology of the Starfish," p. 25 ; "Embryology 

 of Echinoderms. " Mem. of Am. Ac. of Arts and Sciences, N.S. 

 vol. ix. p. 9. 



8 Ueberriie Gattungen der Seeigellarven. Siebente Abhandlung. Kon. 

 Akad. d. Wiss. zu Berlin. Von Job. Miiller, 1855, PL iii. fig. 3. 



