132 INVERTEBRATE PALAEONTOLOGY 



Ordovician types belong to the more primitive section 

 Axonolipa (in which there is no continuous axis in 

 the rhabdosome) ; the more specialized Axonophora 

 persisted into the Silurian. Concurrently with this 

 morphological trait occurs a habit that is readily 

 recognizable. Ordovician types of both sections were 

 either branched (all Axonolipa (PL ix. fig. i), chiefly 

 Arenig and Llandeilo) or " Diprionidian " (e.g. Diplo- 

 graptus, Glossograptus). While both of these modes of 

 growth were retained in the Lower Silurian, " Mono- 

 prionidian" forms (e.g. Monograptus (PI. ix. fig. 2), 

 Rastrites) are especially characteristic of that period. 



(E) ECHINODERMATA 



The Echinodermata, alone of Metazoan Invertebrates, 

 secrete mesodermal skeletons which, like che analogous 

 bones of Vertebrates, are intimately connected with the 

 perishable organs of the body. Hence the phylum is 

 peculiarly well-adapted for palaeontological study. The 

 characteristic reticulate structure of the stereom is as 

 distinctive as that of bony tissue, and makes even 

 obscure fragments of Echinoderms recognizable as such. 

 One class only, the Holothurioidea, is deficient in 

 skeletal tissue; strangely enough remarkably well-pre- 

 served impressions of these forms are known from the 

 Cambrian. Starfish and Brittle-Stars tend to have 

 loosely constructed skeletons, so that their remains, 

 though far from rare, are usually fragmentary. But the 

 massive and coherent tests of most Echinoids, the bulky 

 crowns and extensive columns of many Crinoids, and 

 the compact thecae of Blastoids and advanced Cystids, 

 give ample opportunities for fossilization. 



Morphogeny and ontogeny alike indicate that the 

 ultimate ancestors of the phylum were bilaterally 



