198. 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 Paleaster and Archastertas. In the Upper 
Silurian appeared Palasterina, a genus allied to the living 
A strogonium, etc. 
Cuass II.—ASTEROIDEA. 
Echinoderms with a star-like or pentagonal body, with two or four rows 
of ambulacral feet or tentacles on the 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 hemal systems 
also radiating into the arms. Most of the species bisexual ; the young usually 
passing through a metamorphosis, 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. 
Ordur 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 pedicellarie. 
(Ophiura, Ophioglypha, Ophiolepis, Amphiura, Ophio- 
coma, Astrophyton). 
Order 2. 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 hemal 
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, hemal, and 
nervous systems. 
