ECHINODERMATA. 203 



234. Sea-urchin (Echinus or Arbacid). 



A few skeletons of sea-urchins and sand-dollars will be of great value 

 in enabling the pupil to see how the same general plan of structure may 

 be varied to meet different needs. 



1. Spines (if present) : arrangement and method of attachment. Are 

 they of the same appearance and composition as the skeleton? Do you 

 find any signs of the former presence of ambulacral feet? If so what, 

 and how arranged? Can they all have the same function as in the 

 star-fish? Proofs? 



2. Ossicles : Make out the boundaries. Compare with the condition in 

 the star-fish. What are the special advantages gained by each arrange- 

 ment? Can you find anything corresponding to ambulacral ossicles? 

 (Look for the pores.) What corresponds to the ambulacral groove in 

 Asteriasf Identify the interambulacral ossicles. How arranged? What 

 is radial and what interradial in the urchin? What in the sea-urchin would 

 correspond to the oral and aboral surfaces in the star-fish? Evidences? 

 Find the madreporic body. Make a plot of all the ossicles in this region, 

 noting the differences. Find the genital pores. 



3. "Aristotle's lantern" (the mouth apparatus). 



Examine the structure as a whole. How related to the body? Study 

 the parts in their relation to each other. Number, and method of action? 



DESCRIPTIVE TEXT. 



235. The Echinoderms form a very distinct group of ani- 

 mals, which in the adult condition at least show a decided 

 radial symmetry. They possess a more or less extensive cal- 

 careous exo-skeleton with outwardly directed spines. The 

 star-fishes, sea-urchins, brittle stars, sea-lilies, and sea-cu- 

 cumbers are representatives. They are marine in habit and 

 may be either fixed or slow-moving. They agree with the 

 Ccelenterates in having radial symmetry, and in the absence 

 of a well-marked brain and other signs of cephalization. 

 There is considerable ground for believing that this is an out- 

 come of their sluggish habit, since the larval condition is 

 bilaterally symmetrical, and radial symmetry is clearly adapted 

 to a passive life. It is difficult to determine the relationships 

 of the Echinoderms ; yet it seems probable that their ancestors 

 were bilateral forms. Perhaps they should be considered as 

 connected with the worms rather than with the Ccelenterates. 



236. General Characters. 



i. Larvae are bilaterally symmetrical; in the adult there is 



