120 THE HUMAN SPECIES 



contradiction of the general plan on which the animal body is 

 constructed, for it is exactly on this surface that the division of 

 the two parts of the body is most clearly to be seen. The 

 lowest members of the echinodermata are the asteroidea, in 

 which the anal orifice is absent. 



The arthropoda show certain advances in structure, though, 

 as regards the position of the digestive organ in relation to the 

 other parts of the body, they correspond closely to the worms. 

 An intestine is present passing throughout the whole extent of 

 the abdominal cavity supported by connective tissue, and even 

 by a few muscles. The anal orifice, which is invariably present, 

 is situated either ventrally or terminally. As in the worms, 

 the three distinct parts of the intestine may be clearly dis- 

 tinguished. But the special distinction of this class is their 

 apparatus for seizing and masticating food. This apparatus is 

 placed at the entrance to the alimentary canal, and is formed 

 partly by the borders of the oral aperture, in the form of 

 projections (upper and lower lip), and partly by modifications 

 of the articulated appendages of the body (pseudo-podia, mandi- 

 bles), and is considerably in advance of the masticatory system 

 of the asteroidea, crinoidea and echinoidea. In the crustaceans, 

 whose intestinal canal follows a straight course, the terminal 

 section of the small intestine is transformed into a masticating 

 stomach by means of chitinous margins ; the large intestine is 

 short with appendicular glands and caecal, saccular expansions ; 

 the terminal intestine is quite short with but few csecal pouches. 

 The narrow, small intestine of the arachnida leads into the 

 large intestine, which is usually of relatively great length and 

 provided with lateral caecal sacs ; the terminal intestine is ex- 

 panded. The intestinal region of the myriapoda and insecta 

 differs but little from that already described, except that in 

 the latter the small intestine serves as masticating stomach, 

 or crop, the large intestine is furnished with glands, and in the 

 terminal intestine two distinct sections may be observed, the 

 final one in the form of a rectum. 



The separation of the intestinal canal from the wall of the 

 body, which commences in the worms and arthropoda, char- 

 acterises the mollusca also ; equally clearly defined are the three 

 portions of the intestinal canal, which is coiled in proportion 



