144 MARINE INVERTEBRATES 



crease also by self-division. In this case a constriction is formed, 

 which gradually deepens and forms a complete partition of the 

 body, and two individuals exist where originally there was but one. 



The Actiniaria are carnivorous and very voracious. They feed 

 on small organisms and on shell-fish and crustaceans, which they 

 suck out of their shells. To secure their prey they are armed 

 with an abundant supply of stinging-cells on the tentacles, and 

 also with fine stinging-threads which are ejected from pores 

 which are distributed over the whole body. 



Sea-anemones vary greatly in color and form, and when ex- 

 panded suggest flowers, but do not resemble the one for which 

 they are named. They abound on every shore, the same genus 

 often being found in widely separated regions. They are larger 

 and more highly colored in tropical waters. Many of the species 

 are littoral, and are found in the tide-pools of rocky caverns, 

 on the under side of rocks, arid on the piles of wharves and 

 bridges at low- water mark. The majority of them are attached, 

 but are able to change their location; others, Edivardsia and 

 Cerianthus, swim about when young, and in the adult state burrow 

 in the sand or mud, leaving only their tentacles exposed. Bucid- 

 ium parasiticum is parasitic on the folds of the membrane which 

 hangs from the mouth of the large jellyfish Cyanea arctica. Fetija 

 and Peachia lie on the sea-bottom, with their bodies horizontal 

 like a worm, the mouth-end and tentacles erect. 



Adamsia pdlliata furnishes another example of commensalism ; 

 it lives on the back of shells, commonly the whelk, inhabited by 

 hermit-crabs. Minyas is pelagic ; it has a float at one end, and by 

 means of its tentacles swims about freely. 



A very few species only are described belcw, since these polyps 

 are unmistakable, being always columnar bodies, with the upper 

 disk more or less crowded with tentacles, and so brilliant in col- 

 oring and beautiful in form as to attract attention if found in 

 the expanded state ; otherwise they are easily overlooked. 



GENUS Metridium 



M. marginatum. This is the most conspicuous and abundant sea- 

 anemone of the northeastern coast. It is common from New York 



