V ECHINODERMATA 69 



found amongst Echinoderms, but which, nevertheless, loses any 

 resemblance to this group in its adult stages, developing 

 gill-slits and other features which seem to link it undoubtedly 

 with primitive vertebrate animals. 



Classification of EcJiinodernis mentioned in Chapter V. 



Class I. AsTEROiDEA. — Asterias, the Starfish. 



Solaster, the Sun-star. 

 „ II. Ophidroidea. — Ophiothrix, the Brittle-star. 

 „ III. EcHiNOTDBA. — EcMnus, the Sea-urchin. 

 „ IV. HoLOTHUHlA. — Oucumaria, the Sea-cucumber. 

 „ V. Crinoidea. — Antedon, the Feather Star. 



Pentacrinus, the Sea-lily. 



Practical Work on Echinoderms 



A small Starfish, not more than 2 inches across, and 



In tne j^ Sea-urchin, still smaller, can be quite well kept 

 Aauanum. . , ,. , . ^ 



m the sea-water tank set up according to directions 



given on pp. 49-50. It is very important, however, not to over- 

 crowd the tank ; indeed it is best to keep Echinoderms alone, as 

 they may attack other inmates of the same tank. 



The Rosy Feather Star may be easily obtained by dredging, 

 and it is an interesting and hardy inhabitant of a tank. 



Both Starfishes and Sea - urchins can be fed on little sea 



molluscs, especially bivalves ; they will also eat pieces of raw fish. 



If fed when they are crawling up the glass front of the tank, the 



food can be pushed between them and the glass and their method 



of feeding observed. Careful observations should be made on the 



external structure and habits of the live creatures. 



Dissection The skin covering the upper surface of the arms 



of a of a dead Starfish should be carefully cut away, leaving 



Starfish, intact the small central part bearing the anus and 



madreporite. The form of the alimentary canal from mouth to 



anus can then be exposed by a little displacement of the structures 



which surround it 



The five large digestive sacs should be lifted up to expose the 

 five pairs of reproductive organs which lie below them, but 

 alternate with them in position. The stone canal should be 

 found, and as many points of structure as possible which are 

 mentioned in the preceding chapter should be verified. 



