30 



D. Fix the star-fish, mouth downwards, under water. Select the ray furthest 

 from the madreporic tubercle, and cut through the skin along each side of the ray 

 from the base to the tip ; dissect off this strip of skin, and make a drawing of the 

 organs exposed, viz. •.^■ 



(a) The two large brown sacculated hepatic cseca lying along nearly the whole 



length of the ray on each side of the ambulacral plates ; — notice that near 

 the base of the ray each caecum ends in a delicate tube ; and that the 

 two tubes unite to form a single hepatic duct which opens into the large 

 membraneous pyloric sac of the stomach. 



(b) The stomach, with its eversible cardiac pouches projecting into each ray 



below the hepatic duct ; 



(c) The two reproductive glands, the duct from each passing into the side of the 



inter-radial partitions, where it joins with that from the next ray to open to 

 the exterior in the angle between the rays. 



E. Eeflect the skin from the central disc, taking care not to injure the madre- 

 poric tubercle, and sketch — 



(a) The stomach, with its cardiac and pyloric sacs. 



(b) The short intestine near its centre. 



(c) The rectal glands opening into the intestine. 



(d) The retractor muscles of the cardiac pouches. 

 («) The short oesophagus. 



F. Eemove the pyloric cseca and reproductive organs carefully from the dissected 

 ray, and examine and sketch the ampullae of the pedicels. Scrape off some of 

 these, and notice that the pores for the pedicels are placed between the ambulacral 

 ossicles and alternately near the base and near the ridge, appearing to be in two 

 rows though forming only a single irregular one. 



G. Cut transversely through the ray, and sketch — 



(a) The ambulacral ossicles sloping up to meet in a ridge at the centre, and 



forming the ambulacral groove. 



(b) The small plates — interambulacral plates — on which they rest. 



(c) The inter-radial septa formed by jimction of the inter-ambulacral plates of 



adjacent rays. 



(d) The cut section of the radial water tube lying below the ridge formed by 



the ambulacral plates. 



(e) The radial blood-vessel lying below the water-vessel. 

 (/) The radial nerve lying external to the blood-vessel. 



Dissect out carefully the stone canal which extends from the madreporic 

 tubercle to the circum-oral water tube ; trace one of the radial water vessels to the 

 ring, and notice the sinus running parallel to the stone canal, and opening into a 

 circular vessel beneath the water ring, from which spring the radial blood-vessels. 



