116 ZOOLOGY. 



in diameter ; it is very common from Labrador north- 

 ward. 



Fossil star-fishes allied in most respects to Asterias occur 

 in the Lower Silurian rocks, showing the remarkable persist- 

 •ence of this type of the order. Characteristic Lower Silu- 

 rian forms are Palmaster and Archasterias. In the Upper 

 -Silurian appeared Palasterina, a genus allied to the living 

 Astrogoiiium, etc. 



Class n.— ASTEROIDEA. 



EcMnoderm-s trith a star-like or pentagonaZ body, vith tiro orj/pjr rote* 

 ■of amhulaci'ol feet or tentacles on the oraX side. Body corsered with small, 

 ■sJtort spines, often arranged in groups. The nervous system pentagonal, 

 with nerves extending into Vie arms ; the Water-Toscuiar and hcemal systems 

 -also radiating into the anns. Mont of the species bisexual ; the young usu aUy 

 j)assing through a metamorpJiosis, the star-fishbudding out from the water- 

 •Doscular system of the pluteus, bipinnaria or brachiolaria form, ichich pre- 

 mnidy passes through a morula, gasirula, and cephalula stage. 



■Order 1. Ophiuridea. — Arms round, startiag suddenly from a round, 

 disk-like body. Ambulacra! furrow covered by a series of 

 ventral plates, so that the tentacles or ambulacral feet are 

 thrust out laterally. The ovaries and stomach not extend- 

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

 (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 cceca, as well as the radial nerves and radial haemal 

 and water-vascular canals. A deep ambulacral furrow, 

 containing two or four rows of ambulacral feet or tenta- 

 cles, those at the extremity of the arms without suckers 

 (Brisinga, Ctenodiscus, Luidia, Astropecten, Oreaster, As 

 trogonium, Pteraster, Solaster, Crossaster, Cribrella, Pyc- 

 nopodia, Asterias). 



Lnhnraiory Work. — The larger star -fishes 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, hseinal, and 

 nervous systems. 



