. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 148 MANUAL OF ZOOLOGY. lated, perforate plate on the proximal side of the preceding The centre of the suctorial disc is perforated by an aperture conducting to the interior of the ambulacral tube- ; This perforation of the suctorial discs of the ambulacra, though affirmed by Valentin, is denied by Miiller; and it is 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
. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 148 MANUAL OF ZOOLOGY. lated, perforate plate on the proximal side of the preceding The centre of the suctorial disc is perforated by an aperture conducting to the interior of the ambulacral tube- ; This perforation of the suctorial discs of the ambulacra, though affirmed by Valentin, is denied by Miiller; and it is 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 stmcture, known as " Aristotle's ; The mouth con- ducts by a pharynx and a tortuous oesophagus to a stomach,. Fig. 41.—^Morphology of Echinoidea. i. Echinid larva, a Mouth; h Stomach; c Intestine; s Skeleton. 3. Diagram of Echinus. The spines and the amhulacra are represented over a small portion of the test; the vascular system is cross-shaded ; the nervous system is represented by tlie black line, a Anns ; b Stomach; c Mouth ; d ^nAf Vascular rings round the alimen'tary canal; e Heart; g Test; h Nervous ring round the gullet; i Ambulacral ring or " circular canal" round the gullet; kk Polian vesicles; /Sand canal; tn m Radiating ambulacral canal; n Secondary ambulacral vesicles; o Ambulacral tubes, or " tube-feet"; p Spines; r Madreporiform tubercle. 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 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 " ; Some- times, however, the anal ape
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