ANNULOIDA : ECHINODERMATA. 



161 



tube-foot, shortly after its origin, gives rise to a secondary 

 lateral branch, which terminates in a vesicle. These vesicles 

 or " ampullae " are provided with circular muscular fibres, by 

 the contraction of which their contained fluid is forced into 

 the tube-feet, which are thus protruded. Retraction of the 

 ambulacral tubes is effected by proper muscular fibres of their 

 own, which expel again the fluid which has been forced into 

 them by the vesicles. According to Owen, the terminal 

 sucker in each tube-foot of the Echinus is " supported by a 

 circle of five, or sometimes four, reticulate calcareous plates, 

 which intercept a central foramen, and by a single, delicate, 

 reticulated, perforate plate on the proximal side of the preced- 

 ing group. The centre of the suctorial disc is perforated by 

 an aperture conducting to the interior of the ambulacral tube- 

 foot." This perforation of the suctorial discs of the ambulacra, 

 though affirmed by Valentine, 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. 



jrjg. 57 Morphology of Echinoidea. i. Echinid larva, a Mouth ; b Stomach ; c In- 

 testine ; s 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 / Vas- 

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

 the gullet ; i Ambulacral ring or "circular canal" round the gullet ; k k Pohan 

 vesicles ; / Sand canal ; m m Radiating ambulacral canal ; n Secondary ambula- 

 cral vesicles ; o Ambulacral tubes, or "tube-feet;" p Spines; r Madreponform 

 tubercle. 



The digestive system of the Echinus consists of a mouth, 

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

 perforate five triangular pyramids, the whole forming a 

 singular structure, known as " Aristotle's Lantern." The 

 mouth conducts by a pharynx and a tortuous oesophagus to a 

 stomach, opening into a convoluted intestine, which winds 



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