THE POLYPS AND C0BAL-MAEER8. 



33 



with a large flat stone for the attachment of the sea- 

 anemone. By placing a green sea-weed attached to a stone 

 in the jar, and filling it with sea-water, the animal may be 

 kept alive a long time. 



After observing the movements of the crown of tentacles as they are 

 thrust out or withdrawn, and the eye-spots at the base of some of the 

 tentacles, specimens may be killed expanded by the gradual introduc- 

 tion of fresh water, or by plunging them into picric acid. They should 

 then be transferred to the strongest alcohol, and allowed to soak in it 

 for two or three days until the tissues become hard enough to cut well. 

 Then vertical and transverse sections may be made with a sharp knife. 

 The first fact to observe is that the animal has an alimentary canal, 

 there being a distinct digestive sac (s), separate from the body-walls, 

 hanging suspended from the mouth-opening, and held in place by six 

 partitions which divide the body-cavity into a number of chambers. 

 The digestive sac is not closed, but is open at the bottom of the body, 

 connecting directly with the chambers, 

 so that the chyme, or product of digestion, 

 passes dov^n to the floor of the body, and 

 then into each of the chambers. Fig. 31 

 shows at the base of the body the free 

 edges of the partitions {m) of different 

 heights, with the spaces between them 

 through which the chyme passes into 

 the body- cavity. For the complete pas- 

 sage of the circulating fluid the six 

 primary partitions are perforated by a 

 large orifice ipp) more or less oval or 

 kidney-shaped in outline. The diges- 

 tive sac is divided into two divisions, the 

 mouth and stomach proper, the latter Fie. 

 when the animal is contracted being 

 much shortened, and with the walls 

 vertically folded, as seen in the cut. 



In the tentacles are lodged the lasso- 

 cells, and the tentacles are hollow, com- 

 municating directly with a chamber or 

 space between the partition, and are open at the end. When a pass- 

 ing shrimp, small fish, or worm comes in contact with these tentacles, 

 the lasso-threads are thrown out, the victim is paralyzed, other ten- 

 tacles assist in dragging it into the distensible mouth, where it is partly 

 digested, and the process is completed in the second or lower division 

 3 



31 . — Partly diagi-ammatio 

 sketch of the anatomy_ of an 

 Actinia (Metridium) with the 

 tentacles disproportionately en- 

 larged, s, throat ;-m, mesen- 

 teries, or partitions; o, egg- 

 gland ; cr, mesenterial filaments; 

 e, eyes; op^ orifice through the 

 septa. 



