116 ZOOLOGY. 



in diameter ; it is very common from Labrador north- 

 ward. 



Fossil star-fishes allied in most respects to Aster ias occur 

 in the Lower Siluriaa rocks, showing the remarkable persist- 

 ence of this type of the order. Characterisbic Lower Silu- 

 rian forms are P alabaster and Archasterias. In the Upper 

 Silurian appeared Palasterina, a genus allied to the living 

 Astrogonium, etc. 



Class II.— ASTEKOIDEA. 



EoMnoderms with a sta/r-likeor pentagonal body, with two orjo^ir row» 

 of ambulaeral feet or tentacles on tJie oral side. Body covered with small, 

 slwrt spines, often a/rranged in groups. The nervous system pentagonal, 

 with nerves extending into the arms ; the water-vascular and haanal systems 

 also radiating into the arms. Most of tTte species bisexual; tlie young usually 

 ■passing through a metamMphosis, the star-fish budding out from the water- 

 vascular system ofthepluteus, Upinnaria or brachiola/ria form, which pre- 

 mmsly passes through a morula, gastrula, and cephalula stage. 



Order 1. Ophiuridea. — Arms round, starting suddenly from a round, 

 disk-like body. Ambulaeral furrow eovered by a series of 

 ventral plates, so that the tentacles or ambulaeral feet are 

 thrust out laterally. The ovaries and stomach not extend- 

 ing into the arms ; no anal-opening, no pedicellarise. 

 (Ophiura, Ophioglypha, Ophiolepis, Amphiura, Ophio- 

 coma, Astrophyton). 



Order 3. Asteridea. — Body star like, the arms being gradual extensions 

 of the disk, and containing the reproductive glands, di- 

 gestive coeca, as well as the radial nerves and radial haemal 

 and water-vascular canals. A deep ambulaeral furrow, 

 containing two or four rows of ambulaeral feet or tenta- 

 cles, those at the extremity of the arms without suckers 

 (Brisinga, Ctenodjscus, Luidia, Astropecten, Oreaster, As 

 trogonium, Pterasier, Solaster, Crossaster, Cribrella, Pyc- 

 nopodia, Asterias). 



Ladoratory Work. — The larger starfishes are easily dissected ; the 

 general relations of the integument may be perceived by making 

 transverse and longitudinal sections, while the viscera may be studied 

 by splitting the body and arms in two vertically. The smaller Ophiu- 

 rans can be hardened in alcohol, and stained sections made for 

 studying the intricate relations of the water-vascular, haemal, and 

 nervous systems. 



