198 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 Silurian rocks, showing the remarkable persist. 

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

 rian forms are Palceaster and Archasterias. In the Upper 

 Silurian appeared Palasterina, a genus allied to the living 

 Astrogonium, etc. 



CLASS II. ASTEROIDEA. 



Echinoderms with a star -like or pentagonal body, with two or jour row* 

 of ambulacral feet or tentacles on tJie oral side. Body covered with smalt, 

 short spines, often arranged in groups. The nervous system pentagonal, 

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

 also radiating info the arms. Most of the species bisexual; the young usually 

 passing thromjh a nu '/nnorphosis, the star-fish budding out from the water- 

 vascular system of the pluteus, bipinnaria or brachiolaria form, which pre- 

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



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

 disk-like body. Ambulacral 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, Astrophytou). 



Order 2. Asteridea. Body star-like, the arms being gradual extension? 

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

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

 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). 



Laboratory 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, haemal, and 

 nervous systems. 



