CHAPTER XXXVI 



PHYLUM ECHINODERMATA 



Echinoderms are sharply distinguished from all other animals by 

 several characteristics. One of these is a secondary radial symmetry 

 which does not entirely mask a primitive bilateral condition. If a line 

 is drawn through the interradius of a starfish on which the madreporite 

 lies and is continued along the radius of the opposite ray, the animal will 

 be seen to be divided into two similar halves. The two rays adjacent to 

 the madreporite have been called the hivium and the three nonadjacent 

 rays the trivium. Other characteristics show retrogression, and still 

 others specialization. 



231. Retrogression. — Retrogression may be defined as the possession 

 by an animal of a lower grade of organization than that which has been 

 attained by its ancestors — in other words, the animal has gone backward 

 in its development. This is not to be confused with degeneration, which 

 is simply the loss of characteristics. The cestodes, for instance, show a 

 loss of the alimentary canal due to degeneration. The echinoderms also 

 show a change in the alimentary canal, but instead of disappearing it 

 tends to return to a condition seen in animals with a gastrovascular 

 cavity. Retrogression should not be confused, on the other hand, with 

 the simplicity which belongs to animals lower in the evolutionary scale. 

 This mistake was for a long time made by zoologists in grouping echino- 

 derms with the coelenterates under the term Radiata, because they were 

 seen to possess radial symmetry. Evidences of the fact that echinoderms 

 are much higher in the scale of animal life than are coelenterates are seen 

 not only in many details of the adult structure but also in the fact that 

 the larvae show indications of an advanced condition. 



It is this evidence from the larva which leads to the view that echino- 

 derms illustrate retrogression, which is shown in the following ways: (1) 

 The larva shows bilateral symmetry, while the adult exhibits a secondary 

 radial symmetry; (2) the former possesses an alimentary canal, but in 

 starfishes the anal opening nearly or quite disappears and the enteron 

 structurally and functionally tends to return to the condition of a gastro- 

 vascular cavity; (3) the nervous system in the larva promises a develop- 

 ment higher than any type studied up to this time, but in the adult it 

 acquires a character not greatly in advance of the coelenterates, possess- 

 ing little evidence of centralization. 



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