CHAPTER VI. 

 STARFISH, SEA-URCHINS, AND SEA-CUCUMBERS. 



WE now come to more complicated animals than the 

 jelly-fish, and in which the body is star-like, the parts ra- 

 diating from the middle, forming either five arms, as in the 

 starfish, or five wedge-shaped portions, as in the sea-urchin. 

 As the body is more or less covered with spines or prickles, 

 they are called Echinoderms, or spiny-skinned. 



The student may first examine a common starfish (Asterias vutgaris), 

 which is the most common and accessible Echinoderm to be found on 

 the New England shores. A starfish may be placed in some sea- 

 water and its motions watched. It will be seen to move by thrusting 

 out the numerous feet or suckers by which it pulls or warps its 

 clumsy body over the mussel-beds, rocks, or weeds, the arms being 

 capable of slightly bending. The eyes are the little red spots at 

 the end of each arm or ray. The numerous spines are attached by a 

 sort of ball-and-socket joint to the calcareous framework of the body- 

 walls. 



First, as to the calcareous crust or external support of the starfish. 

 In order to study this, a transverse section should be made through an 

 arm, and a vertical one through the body and along the middle of a 

 single arm, and finally the animal should be divided into two halves, 

 an upper and lower. It will then be seen that the calcareous framework 

 or so-called skeleton consists of a great number of limestone plates or 

 pieces attached by a tough membrane and covered by the skin. Be- 

 tween the plates are small openings by which the water enters the 

 body-cavity. These plates are arranged so as to give the greatest 

 strength and lightness to the body. The mouth is situated on the un- 

 der side in the middle, at the base of the arms. Each arm or ray is 

 deeply channelled by a furrow containing four rows of suckers or 

 " ambulacral feet," which are tentacle-like protrusions of the skin, 

 growing out through orifices in the ambulacral plates, and are a con- 

 tinuation of the water-sacs or " ampulla?" within. The " madreporic 

 plate" is a flattened hemispherical, red, sieve-like body situated on the 

 disk between two of the arms. 



