208 MARINE INVERTEBRATES 
tangle, and often hundreds are brought up in one haul and are 
then killed by steaming. 
The anatomical system of the starfish is easily traced by cut- 
ting off, with scissors, the skin from the dorsal surface of the 
disk, and also cutting a slit down one of the arms. In the center 
of the body will be seen the upper part of the stomach, a small 
star-like spot, from which radiate five branches, which divide and 
lead into the much-ramified, plume-like organs which extend 
through the arms. These are the pyloric ceca, whose function is 
to secrete the digestive fluids. The much-folded stomach occu- 
pies the greater part of the central space, its large convolutions 
showing on top. The mouth connects with the stomach on the 
ventral side. By turning back the skin from the arm, small fili- 
form processes can in some large species be seen; these are the 
dermal branchie, or breathing-organs, which may either project 
through pores in the skin between the ossicles or be entirely re- 
tracted. If the pyloric cwca be laid back, one can see the rib- 
like arrangement of the ambulacral ossicles, and also the ampulle, 
which lie between the ossicles on each side of the ridge. If an 
ampulla be inflated and then pressed, the corresponding tube-foot 
will be seen to extend. To follow successfully the water-vascular 
system, beginning in the madreporic plate and extending through 
the stone-canal to the ring-canal around the mouth and thence 
through the arms, requires a strong glass and more skilful manip- 
ulation. 
ORDER PHANEROZONIA 
FAMILY PORCELLANASTERIDE. 
Genus Ctenodiscus (Plate LI) 
C. corniculatus. Pentagonal; about two and a half inches in diam- 
eter; the body flat, with wide marginal plates, giving a flat edge to the 
rather long, pointed arms ; madreporic plate large; color greenish ; am- 
bulacra without suckers, seeming to be adapted to pushing through soft 
mud rather than dragging over hard surfaces. It lives in deep water on 
muddy bottoms and ranges from Massachusetts to Greenland. When 
dredged the animal is usually found to be filled with soft mud. 
FAMILY ASTROPECTINIDE 
In this family there are only two rows of tube-feet; the rays 
end in sharp points and have large ossicles on the margins. 
