CHAPTEE XXI 



ECHINODERMATA {CONTINUED) : DEVELOPMENT AND PHYLOGENY 



In Chapter XVI. it was stated that whilst a more or less perfectly 

 developed radial symmetry was one of the characteristic features 

 of the phylum Echinodermata when in the adult condition, yet 

 in the immature or larval condition the members of the group 

 have a strongly marked bilateral symmetry. In this feature 

 larval Echinodermata resemble the other Phyla of the animal 

 kingdom which have a well-developed coelom, such as Annelida, 

 MoUusca, Vertebrata, etc. Since, then, the peculiar radial 

 symmetry is gradually acquired during the growth of the 

 Echinoderm, we may possibly discover by a close scrutiny of the 

 life-history what is the nature and meaning of this departure 

 from the ordinary type of structure among coelomate animals. 



There are two kinds of development met with amongst 

 Echinodermata, which may be roughly characterised as the 

 ^' embryonic " and the " larval " type respectively, although 

 neither description is exact. In developmental histories of the 

 first type so much reserve material is laid up in the egg in the 

 form of food-yolk that the young animal whilst in the bilateral 

 stage requires little or no food. In some cases, however, 

 as in AmpMura squamata, the mother pours out a nourishing 

 ■exudation ; but whether this is so or not, the parent in nearly 

 every case carries the young about with her until they have 

 reached the adult condition. In some Asteroidea, as for instance 

 in the Antarctic species Asterias spirabilis (Fig. 280 ), the young 

 become fixed to the everted lips of the mother ; in Amphiura 

 squamata and some other Ophiuroidea the eggs remain in the 

 genital bursae, which serve as nurseries ; in some Spatangoidea, 



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