110 THE EVOLUTION OF CONTINUITY 



a suggestion to be developed further in connection with 

 Segmental Individual evolution. And we would attribute 

 the main peculiarities of the Starfish's structure, and that 

 of Echinoderms generally, to two special modifications, 

 namely, the complete closure of the ancestral medusoid 

 " open coelom " in the region of the velum, and the forma- 

 tion of an anus. 



If during development such a plan as the medusoid's 

 were modified by the opening of an anus at the dome of the 

 bell, and, as a result, the cutting off of the radial canal 

 system from the alimentary tract ; and if at the same time 

 the wide bell-mouth and velum of the medusoid were to 

 develop in a contracted form, tightly enclosing the gullet- 

 region, the circular canal and nerve-cord being relatively 



Fig. 40. — To compare with Fig. 39. a, the partially flattened 

 plan of the sea-urchin ; e, this plan drawn with the radial 

 canals fully extended horizontally, and representing roughly the 

 section of a Starfish. Letters as in previous figure. 



small rings, the fundamental plan of Echinoderms would 

 be the result (Fig. 39). 



A plan such as this, if compressed or flattened in develop- 

 ment, with extension horizontally of the radial canals, 

 would give the rough plan of the Starfish, as in Fig. 40. 

 These figures, it is needless to say, are not offered as actual 

 stages in development, but as illustrating how the medusoid 

 plan is recognisable as the forerunner of the echinoderm 

 plan. 



From the fact that echinoderm larvae are bilaterally 

 symmetrical, and show signs of segmentation, we might 

 reasonably conclude that the mature organism has had a 

 serially-medusoid primitive ancestry such as can be attri- 

 buted to the earthworm, and we might take the bilateral 

 symmetry as indicating that the echinoderm had an ancestor 

 which was free-swimming. This last suggestion is, however, 



