384 



NOTES. 



3o The few animals in which the digestive cavity is wanting are called 

 agastric, and agree in having a very simple structure. Such are some Ento- 

 zoa (as Tape-worm) and unicellular Protozoa (as Gregarina). They absorb 

 the juices, already prepared, by the physical process of endosmose. There 

 are other minute organisms which seem to be able to extract the necessary 

 elements, C H O N, from the medium in which they live. 



36 The cavity of a Sponge is perhaps homologous with the digestive cavity, 

 but is not functionally such. Each cell lining it does its own digestion, tak- 

 ing the food from the water circulating in the cavity. 



37 ''Nothing is more curious and entertaining than to watch the neatness 

 and accuracy with which this process is performed. One may see the rejected 

 bits of food passing rapidly along the lines upon which these pedicellariae 

 occur in greatest number, as if they were so many little roads for the con- 

 veying away of the refuse matters ; nor do the forks cease from their labor 

 till the surface of the animal is completely clean and free from any foreign 

 substance." — Agassiz's Sea-side Studies. 



38 In the larva of the Bee, the anal orifice is wanting. 



39 The length of the canal in Insects is not so indicative of the habits as in 

 Mammals. Thus, it is nearly as long and more complicated in the carnivo- 

 rous Beetles than in the honey-sipping Butterflies. 



40 The object of this is unknown. It does not occur in the Oyster. 



41 In the Nautilus, this is preceded by a capacious crop. 



42 In the Shark, this is impossible, owing to a great number of fringes in 

 the gullet hanging down towards the stomach. 



43 At the beginning of the large intestine in the Lizards (and in many Ver- 

 tebrates above them, especially the vegetarian orders), there is a blind sac, 

 called ccecum. 



* 44 The Crocodile is said to swallow stones sometimes, like Birds, to aid 

 the gastric mill. 



45 In the crop of the common Fowl, vegetable food is detained sixteen 

 hours, or twice as long as animal food. The Dormouse, among Mammals, 

 has an approach to a crop. 



46 In Invertebrates, the gizzard, when present, is situated between the crop 

 and the true stomach ; in Birds, it comes after the stomach. 



47 The Tape-worm has no digestive apparatus, but absorbs the already di- 

 gested food of its host. This is no exception to the rule. The chemical 

 preparation of the food has preceded its absorption. 



48 We find the most abundant saliva in those Mammals that feed on herbs 

 and grain, but its action on starch is extremely feeble. 



49 It is probable that the digestive part of the alimentary canal in all 

 animals manifests a similar mechanical movement. It is most remark- 

 able in the gizzard of a fowl, which corresponds to the pyloric end of 

 the human stomach. This muscular organ, supplying the want of a mas- 

 ticatory apparatus in the head, is powerful enough to pulverize not only 

 grain, but even pieces of glass and metal. This is done by two hard 

 muscles moving obliquely upon each other, aided by gravel purposely swal- 

 lowed by the bird. The grinding may be heard by means of the stetho- 

 scope. 



