114 LOWER INVERTEBRATES. 



marginatum, our New England Actinia, has been chosen to illustrate certain of the 

 important general features in the anatomy of the group. 



Metridium, is common almost everywhere on the New England coast in sheltered 

 pools left by the tides, on spiles of bridges, and on rocks near low-water mark. Under 

 the name of sea-anemone it is known to collectors of marine curiosities as the common 

 zoophyte from Eastport to New York. In no place have I seen the species larger than 

 on the spiles of Beverly Bridge and at Nahant, but equally giant specimens have 

 probably been taken from other localities. The diameter of the largest Metridium. 

 found in the former locality measured^ when expanded with water in its tissues, a little 

 over ten inches in diameter. Forms of Actinaria allied to Metridium from the Florida 

 Keys and Bermuda attain a gigantic size, often fifteen to eighteen inches in diameter. 

 When the Actinia is seen from one side it will be found to have a cylindrical body 

 firmly fixed at one end to some foreign object, and bearing at its free end a circle of 

 tentacles surrounding a central mouth. The tentacles are thickly set together, are 

 very movable, and when the animal is alarmed are quickly drawn to the body. The 

 whole body and the tentacles are very much inflated Avith water, which at the will of 

 the animal can be expelled from the body through the mouth, the body walls, and the 

 tips of the tentacles, where there are small orifices. When inflated with water the 

 body and its appendages are all expanded, but when this water is expelled the animal 

 shrinks to a shapeless lump, the tentacles are drawn back, and there is little resem- 

 blance to its former condition. 



The internal structure of Metridium is of a character typical for most of the Acti- 

 nozoa. If we make a horizontal section through the body about one-half the distance 

 between its attached and free extremities the cross-section thus made will present 

 the following characters. In the centre lies a cavity, the stomach, whose wall is 

 held in position by radiating partitions passing from it to the outer walls of the 

 body. Intermediate between these partitions there are radiating walls or septa which 

 arise from the outer body walls but do not extend to those of the stomach. The body 

 walls from which these partitions all arise are, as a general thing, thicker than those 

 forming the partitions, and in their sides there are openings through which the water 

 at times leaves the body cavity. 



The method by which the Metridium feeds is very simple. The food captured by 

 the tentacles when the anemone is expanded is passed from one member to another 

 through the mouth into the stomach. Here digestion takes place, and. after the soft 

 portions have been digested the harder parts, skeletons, shells, and the like, are thrown 

 off again through the mouth by which they entered the stomach. The fluid passes 

 from the stomach through an opening opposite the mouth into the body cavity, bath- 

 ing the interior of all the organs which lie in that place. 



Special organs of respiration are not unknown among genera of Actinaria allied to 

 Metridium, but in this genus pi-obably the whole external and internal surfaces of the 

 body contribute their part in the performance of this function. In Metridium special 

 organs of sensation are of a very low grade of organization and of the simplest kind, 

 as would naturally be expected from the attached life of the animal. 



Reproduction among the Actinozoa presents some of the most interesting features 

 connected with these animals, and in the case of the coral colonies in which the reef- 

 builders live, is the most important factor in the determination of the ultimate form. 



Three kinds of reproduction, which are known as generation by fission, by gem- 

 mation, and by the laying of eggs, or ovarian, occur in the Actinozoa. The first two 



