b MARINE ANIMALS OF MASSACHUSETTS BAY. 



passing first into the digestive ca\'ity, thence through the opening 

 in the bottom into the main chambered cavity, where it enters 

 freely into all the chambers, and from the chambers into the ten- 

 tacles. The rejected portions of the food, after the process of 

 digestion is completed, return by the same road and are thrown 

 out at the mouth. 



These general features exist in all Polyps, and whether they 

 lead an independent life as the Actinia, or are combined in com- 

 munities, like most of the corals and the Halcyonoids ; whether the 

 tentacles are many or few ; whether the partitions extend to a 

 greater or less height in the body ; whether they contain limestone 

 deposit, as in the corals, or remain soft throughout life as the sea- 

 anemone, — the above description applies to them all, while the 

 minor differences, either in the tentacles or in the form, size, color, 

 and texture of the body, are simply modifications of this structure, 

 introducing an infinite variety into the class, and breaking it up 

 into the lesser groups designated as orders, families, genera, and 

 species. Let us now look at some of the divisions thus estab- 

 lished. 



The class of Polyps is divided into three orders, — the Halcy- 

 onoids, the Madreporians, and the Actinoids. Of the lowest 

 among these orders, the Actinoid Polyps, our Actinia or sea-ane- 

 mone is a good example. They remain soft through life, having 

 a great number of partitions and consequently a great number of 

 tentacles, since there is a tentacle corresponding to every cham- 

 ber. Lideed, in this order the multiplication of tentacles and 

 partitions is indefinite, increasing durmg the whole life of the 

 animal with its growth ; while we shall see that in some of the 

 higher orders the constancy and limitation in the number of these 

 parts is an indication of superiority, being accompanied by a 

 more marked individualization of the different functions. 



Next come the Madreporians, of which our Astrangia, to be 

 described hereafter, may be cited as an example. In this group, 

 although the number of tentacles still continues to be large, they 

 are nevertheless more limited than in the Actinoids ; but their 

 characteristic feature is the deposition of limestone walls in the 

 centre of the chambers formed by the soft partitions, so that all 

 the soft partitions alternate with hard ones. The tentacles, al- 



