. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . velopment, separates from the stalk and swims freely about by means of its arms. It can attach itself temporarily by tubular pro- cesses, which are developed on the dorsal surface at the point where the stem was attached. The ossi- cles, or plates which cover the dorsal surface, are free, making the crinoid an animal of innu- merable joints. The crinoids are particularlyinteresting from the fact that theyhave existed from early geologictimes, and their history is writtenin sto
. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . velopment, separates from the stalk and swims freely about by means of its arms. It can attach itself temporarily by tubular pro- cesses, which are developed on the dorsal surface at the point where the stem was attached. The ossi- cles, or plates which cover the dorsal surface, are free, making the crinoid an animal of innu- merable joints. The crinoids are particularlyinteresting from the fact that theyhave existed from early geologictimes, and their history is writtenin stone. In the early ages theywere the only class of echino-derms, and their evolution intoother forms can be traced throughsuccessive geologic periods. Theyexisted in such vast numbers thatthe fossil forms are plentiful andare familiar to every student ofgeology, and are known as stone-lilies and encrinites. To-day theyare decadent, there nowremainingonly twelve of the two hundred geiiebed of crinoids is found off Cuba, on the slope of the roast where thewater rapidly deepens from one hundred to two hundred \vliidi existed formerly. A fine 236 MARINE INVERTEBRATES Although the crinoids are deep-water forms, and are neverfound on the beach, they are given here because they follow inthe system of classification and illustrate another curious class ofechinoderms. The brief description given may add interest to thefossil forms to be seen in museums. VI ARTHROPODA TABLE SHOWING THE CLASSIFICATION OF THE ARTHROPODSDESCRIBED IN THIS CHAPTER Phylum ARTHROPODA ClassCRUSTACEA SubclassEntomostraca (Crustacea of small, often of almost microscopic minuteness, and of comparativelysimple organization; appendages have little differentiation) Order PHYLLOPODA(Appendages resemble leaves; mostly fresh-water species) Suborders Genera EUPHYLLOPODA (Phyllopoda of considerablesize, bearing ten to sixtypairs of leaf-like sivim-ming-feet) CLADOCERA (Small phyllopoda; bodylaterally compressed; ca
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