592 SPECIAL PHYSIOLOGY. 



other animals, there is no alimentary canal, nutriment being absorbed by them 

 directly, through the integuments, from the digested food, or from the juices 

 of the animal in which they live. In the tape-worms, straight tubes, with 

 transverse or even radiating branches, exist, which are doubtless concerned in 

 the nutritive processes, rather as circulatory and respiratory, than as digestive 

 organs. 



In the Echinodermata, the alimentary canal is well developed, distinct from 

 the walls of the body, provided, in most cases, with openings at both extremi- 

 ties, and even supported by a mesenteric fold. In the Crinoida, the stomach 

 and intestine are situated in the central part of the body, the latter opening at 

 one side. Below the complex masticatory apparatus, elsewhere described (p. 

 581), the intestine of the Echinida, at first narrow, widens out, and presents a 

 csecal dilatation, beyond which the intestine coils twice round within the shell, 

 reversing its direction in the latter half. In the Star-fishes, and also in the 

 Ophiurida, the alimentary canal is very short, and gives oft" two ramified diver- 

 ticula into each ray ; the intestine opens by a minute orifice on the back ; un- 

 digested matters are frequently discharged by the mouth. In the holothurida, 

 the intestine describes a zigzag course ; the outlet is placed at the higher end 

 of the body. 



Ccelenterata. In this well-defined aquatic Subkingdom, there is no longer 

 an alimentary canal, separate from the walls of the body, and provided with 

 an oral and anal aperture. The digestive canal is very short and wide, and 

 has but one external opening, the mouth, which, however, serves both for the 

 ingestion of food, and the egestion of residual matters, and excretions ; at its 

 inner end, the digestive canal opens widely into the general cavity of the body. 

 From this latter, numerous canaliculi are prolonged, in the medusae, into the 

 disc, some of them opening by pores in its margins. In certain ctenophorous 

 forms, as in beroe, cydippe, and csestum, the body cavity also opens, by one 

 or two orifices or pores, at a point opposite to the oral aperture, but these are 

 not intestinal or anal openings. 



In the Actinozoa, the digestive canal projects a certain distance into the 

 body cavity, which forms, outside that canal, the perivisceral cavity. In the 

 Hydrozoa, the digestive canal becomes continuous, by a very wide opening, 

 with the body cavity, without any portion of it projecting into that chamber ; 

 hence there is no surrounding or perivisceral chamber, and the outer surface 

 of the continuous digestive and body cavities, are both in contact with the 

 water. The hydra may be inverted, like the finger of a glove, and its outer 

 surface, now become internal, will digest its food equally well. 



In the compound Hydrozoa, the lower end of the body cavity of each polyp 

 communicates, by a tubular process, with a common channel extending 

 through the entire stem, a circulation of fiuid, often containing granular par- 

 ticles, taking place through the whole colony. In the compound Actinozoa, 

 the digestive cavities of the individual animals also open into a chamber in the 

 common fleshy basis, the aperture being radiate in shape, and capable of being 

 closed by muscular contraction. 



Protozoa. Of these, the higher Infusoria alone possess any representative 

 of an alimentary canal. In the paramecium, for example, a depression exists 

 on the surface of the body, bordered by cilia, and leading to an aperture called 

 the mouth, from which a short blind tube, named the gullet, dips into the 

 sarcodous body. This is the last imperfect trace of a digestive canal seen in 

 the Animal Kingdom. Temporary cavities, formed by movements in the 

 sarcode, into which food or coloring matters may penetrate, appear like stom- 

 achs, whence the name polyyastric applied to some of those microscopic crea- 

 tures ; but as these cavities may be seen slowly to move within the sarcode, 

 up one side, and down the other, they are no longer regarded as stomachs. 

 The undigested food, after thus circulating through the sarcode, is expelled at 

 a particular point, either near the mouth, or near the hinder end of the body, 

 which point is only then recognizable. 



In the Sponges, Rhizopods, and Gregarinida, no trace of an alimentary 

 canal exists. The system of canals with incurrent and excurrent apertures, 

 seen in the Spongida, is not digestive more than it is respiratory or reproduc- 

 tive, but depends on the plan of construction of Sponges, which are composed 



