EC HI NO DERM A TA . 



479 



FIG. 271. A. THE LAEVA OP A HOLO- 



THDROID. B. THE LAKVA OF AN AsTEKIAS. 



m. mouth ; st. stomach ; a. anus ; I.e. 

 primitive longitudinal ciliated band; pr.c. 

 pras-oral ciliated band. 



The first group is characterised by the growth of a number of 

 arms more or less surrounding the mouth, and supported by cal- 

 careous rods. The ciliated band retains its primitive condition as a 

 simple longitudinal band throughout larval life. There is a very 

 small prae-oral lobe, while an anal lobe is very largely developed. 



The Auricularia and Bipinnaria resemble each other in shape, m 

 the development of a large pras- 

 oral lobe, and in the absence of A. 



provisional calcareous rods ; but 

 differ in the fact that the ciliated 

 band is single in Auricularia (fig. 

 271 A), and is double in Bipin- 

 naria (fig. 271 B). 



The Bipinnaria larva shews 

 a great tendency to develop soft 

 arms ; while in the Auricularia 

 the longitudinal ciliated band 

 breaks up into a number of 

 transverse ciliated bands. This 

 condition is in some instances 

 reached directly, and such larvae 

 undoubtedly approximate to the 

 larvae of Antedon, in which the 

 uniformly ciliated condition is 

 succeeded by one with four transverse bands, of which one is pra3-oral. 



All or nearly all Echinoderm larva? are bilaterally symmetrical, 

 and since all Echinodermata eventually attain a radial symmetry, a 

 change necessarily takes place from the bilateral to the radial type. 



In the case of the Holothurians and Autedon, and generally in the 

 viviparous types, this change is more or less completely effected in the 

 embryonic condition ; but in the Bipinnaria and Pluteus types a radial 

 symmetry does not become apparent till after the absorption of the 

 larval appendages. It is a remarkable fact, which seems to hold for 

 the Asteroids, Ophiuroids, Echinoids, and Crinoids, that. the dorsal side 

 of the larva is not directly converted into the dorsal disc of the adult ; 

 but the dorsal and right side becomes the adult dorsal or abactinal 

 surface, while the ventral and left becomes the actinal or ventral 

 surface. ' 



It is interesting to note with reference to the larvae of the 

 Echinodermata that the various existing types of larva? must have 

 been formed after the differentiation of the existing groups of the 

 Echinodermata ; otherwise it would be necessary to adopt the im- 

 possible position that the different groups of Echinodermata were 

 severally descended from the different types of larvae. The various 

 special appendages, etc. of the different larvse have therefore a purely 

 secondary significance ; and their atrophy at the time of the passage of 

 the larva into the adult, which is nothing else but a complicated 

 metamorphosis, is easily explained. 



