172 INVERTEBRATE ZOOLOGY 



to each arm. The ring canal communicates with the exterior 

 by way of a stone canal which opens through the madreporite. 

 In many Asteroidea, there is but a single madreporite and stone 

 canal, but in some two or more of these structures are present. 

 This latter condition is frequently associated with the powers of 

 asexual reproduction. Small tufts of tubules called Tiedemann's 

 bodies, interradial in position, are connected with the ring canal. 

 Within these organs the amoeboid lymph cells which occur in 

 the water-vascular system are formed. In addition, long- 

 stalked vesicles called Polian vesicles jointhe ring canal interradi- 

 ally in some forms of Asteroidea. To these, also, have been 

 ascribed the function of lymph glands. 



The longitudinal canal in each arm passes along the median 

 line of the ventral surface just outside (that is, ventral to) the 

 ambulacral plates. At the tip of each ray, the ambulacral 

 canal ends in a single tactile organ. Most Asteroidea have four 

 longitudinal rows of tube-feet in the ambulacral area of each 

 arm, but in some (e.g., Henricia) there are but two longitudinal 

 rows. By opposite lateral branching, canals are given off along 

 the course of the longitudinal canal and communicate with the 

 tube-feet. In the typical condition of four longitudinal rows of 

 feet, the two lateral branches arising from the same level on the 

 longitudinal canal are of unequal length. This condition usually 

 alternates in adjacent pairs, so on each side of the longitudinal 

 canal a long and a short transverse canal alternate. Thus the 

 tube-feet fall into two parallel rows on each side of the longitudi- 

 nal canal. Each ambulacrum is a muscular tube which, at its 

 inner end, bears a muscular sac called an ampulla. By contrac- 

 tion of the walls of the ampulla, the fluid in the tube-foot is put 

 under compression. Relaxation of the muscles in the foot per- 

 mits the tube to elongate greatly. At the same time, the hydro- 

 static pressure causes a cuplike disc at the end of the foot to 

 flatten, and in case the disc comes in contact with some object it 

 adheres to it. Shortening of the tube is accomplished by allowing 

 the fluid to turn back into the ampulla. When several feet 

 become attached to an object, by their concerted contraction 

 they drag the whole body of the starfish, and in this manner 

 locomotion is accomplished. 



The Sexes and Reproduction. — Though starfishes are of 

 separate sexes, there are no external features which distinguish 

 the sexes. Some species have been found to reach maturity in 



