58 CCELENTERATA AND 



which have a spherical body with no arms. The body is 

 ordinarily spinous, whence the name of the typical genus, 

 Echinus. In Echinarachnius, the " sand dollar" or "sand 

 cake," the sphere is flattened into a thin, slightly conical 

 disk. 



In most of the Holothurians, "sea-cucumbers," the bod^ 

 is columnar; in some vermiform. In this group portions 

 of the body may be covered with scales without prominent 

 spines, but is leathery, or soft and flexible. 



The stellate Echinoderms are distinguished by an oral 

 and an aboral region. The oral region in the star-fishes 

 is situated below; in the Crinoids above, as the animal is 

 ordinarily placed. A mouth is found at or near the centre 

 of the oral region. The vent when present is, in the star- 

 fishes, on the centre of the aboral region. The brittle-stars 

 have no vent. 



The oral surface of the star- fishes is formed of five 

 double rows of plates extending from mouth to extremity 

 of the ray. These plates are called ambulacral plates and 

 from the intervals between them arise the feet which are 

 often with suckers at the free end and with a single or paired 

 inflation or ampulla at the opposite end in the body . These 

 feet are in two or four rows in each ray. In the brittle- 

 stars the ambulacral plates are covered by a ventral series 

 of plates or integument. 



In the spheroidal Echinoderms the aboral surface of the 

 star-fish is reduced to a small circle at the pole opposite 

 the mouth. The ambulacrals appear as meridional rows 

 of plates extending from mouth to aboral circle. In the 

 "sand dollars" a portion of these plates on the upper sur- 

 face is specialized into a rosette of five pairs of plates 

 arranged in a series known as the petaloid region. The 

 position of the anus varies in the sea-urchins from the 

 neighborhood of the mouth to a point on the opposite pole 



