306 EMBRYOLOGY AND CLASSIFICATION. 



the highest class in this division the Echino- 

 derms made an exception to this rule, and did 

 not agree with the other Radiates in its mode of 

 development. Johannes Muller, one of the most 

 eminent investigators of modern times, in a long 

 series of memorable papers upon the Embryology 

 of Radiates, has maintained that the larval con- 

 dition of the young Echinoderm, so far from being 

 homologous with the early stages of development 

 in the other classes, is essentially bilateral. It is 

 true that there is in many of the Radiates some- 

 thing akin to a bilateral symmetry, though it is 

 always subordinate to the prevailing idea of radi- 

 ation in the plan. This tendency is already quite 

 perceptible in the highest order of the Acalephs, 

 the Ctenophorae, and becomes still more so in 

 some representatives of the class of Echinoderms, 

 the highest in this type. The resemblance of the 

 larvae of the Echinoderms to the Ctenophorae had 

 not escaped my notice ; but during the past year 

 my son has shown conclusively, in a series of 

 microscopic investigations not yet published, that 

 they are as truly radiated as the most circular or 

 spheroidal of the type. The further growth of 

 the young Echinoderms, from the young Comatula 

 (as far as its history is known in its pentacrinal 

 condition) to the gradual transformation of the 

 common Star-Fish, with its undivided circular 

 outline, with its two rows of simple ambulacra! 



