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 caeca, 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 branchiae, or breathing-organs, which may either project 

 through pores in the skin between the ossicles or be entirely re- 

 tracted. If the pyloric caeca be laid back, one can see the rib- 

 like arrangement of the ambulacral ossicles, and also the ampullae, 

 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 PORCELLANASTERID.E. 

 GENUS Ctenodiscus (Plate LII) 



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 ASTROPECTIHTDJE 



In this family there are only two rows of tube-feet ; the rays 

 end in sharp points and have large ossicles on the margins. 



