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MANUAL OF ZOOLOGY. 



difficult to believe that it would not impair the functions of 

 the feet in the act of protrusion. 



The digestive system of the Echinus consists of a mouth, 

 armed with five long, calcareous, rod-like teeth, which per- 

 forate five triangular pyramids, the whole forming a singular 

 structure, known as "Aristotle's lantern." The mouth con- 

 ducts by a pharynx and a tortuous oesophagus to a stomach, 

 opening into a convoluted intestine, which winds round the 

 interior of the shell, and terminates in a distinct anus. The 

 mouth is always situated at the base of the test, and may be 

 central, sub-central, or altogether excentric in position. The 



Fig. 39. Morphology of Echinoidea. i. Echinid larva, a Mouth ; b Stomach ; c 

 Intestine ; ^ Skeleton. 



2. Diagram of Echinus. The spines and the ambulacra are represented over a 

 small portion of the test ; the vascular system is cross-shaded ; the nervous system 

 is represented by the black line, a Anus ; b Stomach ; c Mouth ; d and / Vascular 

 rings round the alimentary canal : e Heart ; g Test ; h Nervous ring round the 



ibulacral vesicles ; o 

 abercle. 



anus varies considerably in its position, being usually situated 

 within the apical disc, and surrounded by the genital and 

 ocular plates, when the test is said to be "regular." Some- 

 times, however, the anal aperture is without the apical disc, 

 and is removed to some distance from the genital plates, when 

 the test is said to be "irregular." The convolutions of the 

 alimentary canal are attached to the interior of the test by a 

 delicate mesentery; the surface of which, as well as that of the 

 lining-membrane of the shell, is richly ciliated, and subserves 

 the purposes of respiration. 



The proper blood-vascular system (fig. 39, 2) consists of a 

 central, fusiform, contractile vesicle, or heart. This gives off 

 one vessel, which forms a ring round the intestine near the 



