I40 ECHINODERMATA 



enemies and later regenerate the entire alimentary canal. Mayer 

 (Seashore Life) reports that the brittle sea cucumber Synapta lives 

 in a ringed sand tube agglutinated with slime. Bands of sand are 

 formed and forced down to finally form the tube. Leptosynapta 

 breaks itself up into short lengths and then regenerates. 



Fossil Relatives. — Cystoidea are extinct. Their fossils are con- 

 fined to the Paleozoic, extending from the Cambrian to the Permian 

 with maximum development in the Ordovician and Silurian. The 

 Blastoidea, also extinct, are confined to the Paleozoic, ranging from 

 the Ordovician to the Permian. The Crinoidea range from the 

 Ordovician to the present time, being most abundant as fossils in 

 the Upper Paleozoic. The Asteroidea, Ophiuroidea, Holothuroidea 

 and Echinoidea have descended to the present, from the upper 

 Paleozoic. 



Ancestry and Relationships to Other Phyla. — It has been found 

 so difficult to connect the Echinodermata with other Phyla that they 

 have been for some time considered relatively isolated. Early 

 attempts to link them with the Coelenterata on account of their 

 radial symmetry have encountered the objection that the Echino- 

 dermata have an extensive coelom or body cavity. They have 

 highly developed alimentary and nervous systems not present in 

 the lower Phyla. It is now believed that the embryonic history 

 of the Echinoderms indicates that they developed from a group with 

 bilateral symmetry. The oldest classes of Echinodermata are those 

 with the radial symmetry least developed. Thus the stalked Cri- 

 noids are considered the ancestors of the free living forms. Crinoidea, 

 Cystoidea and Blastoidea represent the primitive type, while radially 

 symmetrical Asteroidea, Ophiuroidea and Echinoidea are probably 

 descended from the primitive Holothuroidea. The Echinodermata 

 are an isolated group, with no living or fossil links. The echinoderm 

 larva, somewhat comparable to that of Balanoglossus, called Tor- 

 naria (see page 217), has led evolutionists to place the Phylum in the 

 series of Invertebrates that are supposed to be progenitors of 

 Vertebrates. 



