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THE SEA URCHIN; Echinus esculentus. 



A. Observe the natural movements of the living animal, noticing that it uses for 

 locomotion not only the pedicels or tube feet, like the star fish, but also the spines 

 which cover the test ; and that on a flat surface it uses its teeth also for this purpose, 

 by projecting them and pushing itself over them. Touch any part of the test, and 

 observe that the adjacent spines and pedicellarise all converge to the point of stimu- 

 lation, the pedicellarise attempting to grasp the irritating object. 



B. Make a drawing of the sea urchin from the side, indicating : — The spheroidal 

 form ; the small tubercles bearing movable spines ; the five ambulacral areas per- 

 forated for the pedicels, and the five inter-ambtdacral areas between them. Notice 

 that the ambulacral areas are wide where they commence at the mouth, and 

 narrow towards the aboral region. 



C- Scrape off the spines from a small portion of both areas, and sketch — 



(a) The double row of plates in each area bearing the tubercles, which are of 



various sizes, and bear corresponding sized spines. 

 (6) The perforations in the ambulacral plates for the pedicels. 



D. Scrape off a few of the spines in the aboral region, and indicate the arrange- 

 ment of the plates exposed : — 



(a) The five genital plates perforated for the genital ducts, one of them larger 



and rougher than the others serving as the madreporie tubercle. 



(b) The five smaller ocular plates placed between the genitals — one at the apex 



of each ambulacral area. 



(c) The central semi-membraneous area surrounded by the genital plates, and 



perforated by the anal opening. 



E- Sketch the oral surface, showing the large membraneous area with the mouth, 

 occupied by the five teeth in the centre ; and the five pairs of tentacles projecting 

 from the sinuses which surround the lantern. 



p. Eemove, examine (low power), and draw the following structures : — 



(a) One of the spines, indicating its fiuted appearance and the cup or socket 



at its base surmounted by a ridge with muscular fibres adhering, which 



held the spine in contact with the tubercle on the surface of the test. 

 (6) Several of the pedicellarise, — noting that they consist of a long slender stem, 



partly calcareous and partly membraneous, and of a head made up of three 



movable mandibles, 

 (c) One of the pedicels with its sucker-like tip containing five or six calcareous 



plates. 



