226 BULLETIN OF THE BUREAU OF FISHERIES. 



united in more or less compact masses, the whole colony being brought into communal 

 relations by means of a common body substance, the cocnenchyme, through which a 

 system of gastric canals ramifies in various degrees, and thus directly or indirectly brings 

 the various individuals into communication, both for nutritive and for protective pur- 

 poses. It is most interesting to obser\'e the sensory continuity which prevails in one of 

 these colonies. Any irritation which disturbs one polyp is promptly conveyed to every 

 individual of the colony, a sensory wave passing over the community not unlike that 

 produced by the wind upon a field of grain or tall grass. 



MORPHOLOGY. 



In form and structure Anthozoa have many features of very considerable interest, 

 not only to the critical student of morphology but to the casual observer or student of 

 natural history. In form there is exhibited a wide range, from the simple polyp, of 

 minute size and of structure hardly more complex than that of a hydrozoan, to the com- 

 plexity and mass of immense communal aggregates of reef corals and sea plumes and 

 gigantic anemones more than a foot in diameter. 



In fundamentals of structure there is of course a more or less intimate homology 

 running throughout the entire class, though with considerable ordinal peculiarities, and 

 even generic and specific features more or less unique. The general form of the individual 

 polyp, whether actinian or alcyonarian, is cylindrical or barrel-shaped, with a hollow 

 gastric interior. Usually the height is considerably greater than the diameter, but 

 owing to the highly contractile powers of the organisms these proportions vary greatly 

 under various conditions of contraction. The body is terminated by a basal portion 

 usually known as the foot, or pedal disk, and by an upper terminal portion, the oral 

 disk, or peristome. The body proper is usually designated as the column. In sedentary 

 species the pedal disk forms an organ of attachment, or in those having motile power 

 constitutes the chief organ of locomotion. This feature is rather peculiar to the free- 

 living species, though it may be absent in such tube-dwelling forms as Cerianthus. 



The oral disk is characterized by the presence of a distinct mouth, which is usually 

 of oval shape, especially in actinians, and with its edges, or lips, more or less folded or 

 corrugated. The mouth communicates with the gastric cavity or enteron through an 

 oesophagus, the inner end of which opens abruptly into the gastric cavity. 



A series of radially arranged septa, the mesenteries, connect the inner walls of 

 the oesophagus with those of the body, thereby forming a regular series of alternating 

 septa and pockets, the latter ending blindly at the oral disk above, except as they 

 may communicate with the bases of hollow tentacles, but opening freely below into 

 the enteron. The margins of the mesenteries below their oesophageal connections are 

 often more or le.ss thickened, and bear filaments which are characteristic of Anthozoa. 

 They are composed of entoderm, and are richly supplied with gland and nettling 

 organs. In many actinians these latter organs are threadlike, highly contractile struc- 

 tures known as acontia, which are loaded with specialized nettling cells, and may be 



