556 INVERTEBRATE MORPHOLOGY. 



gland is formed of loose connective-tissue trabeculre, which 

 are covered by cells frequently found in active division, and 

 are supposed to become the amoeboid corpuscles of the coslorn 

 and blood system. 



The hydroccel consists as usual of an oral ring and five 

 radial canals, the latter lying at the bottom of the ambulacral 

 grooves and therefore external to the ambulacral plates (Fig. 

 254, rh). Between each pair of plates a branch passes upwards 

 (i.e., aborally), and dilates into a globular sac, the ampulla 

 (Figs. 254 and 255, am), which is occasionally double, and from 

 this a cylindrical tentaclelike process passes outwards again 

 between two plates forming extensible processes (Fig. 254, tfy 

 equivalent to the tentacles of the Criuoids. These processes 

 in some of the more primitive forms, such as Luidia and 

 Astropecten, and the terminal ones at the extremity of the arms 

 of all forms, are conical in shape, but more usually the great 

 majority of them are provided at their extremities with suck- 

 ing disks, whereby they can adhere to foreign bodies and serve 

 thus as locoinotor organs. Hence they are known as the 

 tube-feet or ambulacra. In some forms they are arranged in 

 two rows, one on each side of the axis of the arm, but in 

 others, as for example the common Starfish Asterias, the suc- 

 cessive feet of each row alternate with each other, so that they 

 have the appearance of being arranged in four rows. By 

 means of the muscles of the wall of the ampullae water can be 

 forced into the tube-feet, which may be thus extended, a 

 circular valve occurring in the branch which passes from the 

 radial canal to the ampulla preventing the water from passing 

 back into the canal. Contrary to what occurs in the Criuoids, 

 there are several appendages to the oral ring, in addition to 

 the stone-canal. This leaves the ring in the iuterradius CD 

 and, passing aborally, communicates with the axial sinus which, 

 as already stated, opens to the exterior by the madreporite. 

 This is a complicated calcareous sieve-plate of some thickness, 

 and the union of the canal and the sinus takes place within 

 its substance, so that in reality the canal seems to open to 

 the exterior. The embryonic history, and the fact that injec- 

 tions forced through the tubercle pass into both the sinus and 

 the canal, show that what has been described is the true 



