SURVEY OF INVERTEBRATES 83 



The Rotifera, or Wheel-animalcules (Trochelminthes), con- 

 stitute an immense group of tiny animals commonly found in 

 fresh and salt water, in association with the Protozoa. Micro- 

 scopic though they are, they possess a highly complex series of 

 internal organs, in spite of which they can withstand slow drying 

 in mud. In this condition Rotifers may be blown about until they 

 happen again upon moisture, whereupon they gradually 'swell 

 up' and assume active life and reproduction. 



The Bryozoa are frequently referred to as Moss-animals be- 

 cause most of them are mat-like fixed colonies growing on sub- 

 merged objects in sea or pond. A superficial examination of the 

 common fresh-water Plumatella, for example, might suggest that 

 it is a Hydroid, but closer scrutiny reveals a very different struc- 

 tural plan, including complex internal organs. In addition to 

 reproducing sexually and by typical budding, Plumatella develops 

 peculiar internal buds, or statorlasts, enclosed within a chitinous 

 shell. In the event that the pond dries up or the colony is frozen, 

 the resistant statoblasts survive to start a new colony upon the 

 return of favorable conditions. 



The Brachiopoda, or Lamp-shells, at first glance look like 

 Clams, and so bear no obvious resemblance to the colonial Bryozoa. 

 However, Brachiopods are actually related to the Bryozoa, as is 

 evidenced by their internal structure which approaches rather 

 closely to that of Plumatella. Brachiopods constitute one of the 

 older marine groups, their shells constituting conspicuous and 

 characteristic fossils in the more ancient rock strata. Those living 

 to-day are merely little-changed survivors of a successful group 

 of yesterday — perhaps nearly a billion years ago in the early 

 Paleozoic seas. Indeed, the genus Lingula has persisted without 

 change, to be well dubbed the "senior genus of the world of 

 animal life." (See: Appendix I.) 



G. ECHINODERMS 



Everyone who has spent a summer at the seashore is certainly 

 familiar with the Starfish and Sea Urchin, common examples of 

 the marine phylum of spiny-skinned animals, the Echinodermata. 

 All members of the group have radially symmetrical bodies — a 

 condition we have not seen since we left the Coelenterata. How- 

 ever, the symmetry indicates no direct relationship with the 

 Coelenterates, because during early embryonic life an Echinoderm 



