XXI.] ANOMALIES OF DEVELOPMENT. 277 



pseudembryo is, that tlie larva is transformed into the mature 

 form, but the pseudembryo is not so transformed ; its substance 

 is in most cases absorbed by the growing embryo : there is thus 

 a transformation of the substance, but this is not as the sub- 

 stance of a larva is transformed into that of the perfect form ; it 

 is rather as food is transformed into the substance of the 

 organism. But the pseudemhryo of the star-fish, instead of Pecu- 

 being absorbed, is cast off, and continues to live for some days. ^j^^^. ^j 

 Dr. Thomson quotes Dr. Carpenter's remark, that the structures the star- 

 first developed in the egg of the bird hold nearly the same 

 relation to the rudimentary chick that the pluteus (pseudembryo) 

 bears to the incipient Echinus or Ophiura, or the Bipinnaria 

 to the incipient star-fish. 



There are also these distinctions between pseudembryos and Pseudem- 



ordinary larvas : — In the cases now under consideration, deve- }"^y°s ^^^ 

 *' . _ ' larvae. 



lopment is not the criterion of morphology, nor of the true 

 affinities of the species j on the contrary, the pseudembryonic 

 forms are much less constant throughout the class than the 

 mature ones ; and, unlike what we find in metamorphosis and 

 metagenesis, there is no morphological resemblance whatever 

 between the pseudembryonic and the mature forms of the Echi- 

 nodermata. A tadpole and a frog are both Vertebrates ; a maggot 

 and a fly are both annulose animals ; a zoea and a crab are both 

 Crustaceans ; and a Medusa is morphologically a hjdrozoon, 

 though externally much modified ; but a pluteus or a bipinnaria 

 is not in the least like an echinoderm. For these reasons I do 

 not think it likely, or even possible, that the ancestors of the 

 Echinodermata are in any way represented by their present 

 pseudembryos. I am at a loss even to guess how this extra- 

 ordinary mode of development can have originated. 



