204 ZOOLOGY. 



a more or less complete radial arrangement of equivalent parts, 

 usually on the plan of five. In this radial plan all the principal 

 sets of organs share: as the nervous, digestive, reproductive, 

 etc. 



2. There is a complete differentiation of digestive tract and 

 body cavity. 



3. The blood-vascular system is partially differentiated 

 from the body cavity, but communicates with it. 



4. A calcareous exo-skeleton occurs, derived from the meso- 

 derm. It may consist of isolated spicules or united plates. 

 Associated with these are usually spines, from which the 

 group is named. 



5. A water-vascular system, consisting of a series of tubes 

 (closed except at one point), muscular sacs (ampullae) and 

 distensible feet, serves a locomotor and respiratory function. 



6. Reproduction sexual; development usually indirect, i. e., 

 with a metamorphosis. Reproduction by budding does not 

 occur. 



237. General Survey. The majority of echinoderms have 

 a central disc in which is located portions of the various sets 

 of organs. Ordinarily there radiate from this disc more or 

 less clearly defined rays or arms in which lie radial outgrowths 

 from the central organs. The spaces between the rays (inter- 

 radii) may be bridged by growth in such a way that the dis- 

 tinction between rays and disc is not marked. In crinoids the 

 arms may be much branched. The oral-aboral axis is usually 

 pronounced, often short, and is vertical in position (asteroids, 

 echinoids, crinoids, etc.), though in the sea-cucumbers (holo- 

 thuroids) it is horizontal and much elongated. Star-fish are 

 flattened vertically, as are the sand-dollars, but many of the 

 urchins (echinoids) are dome-shaped. The antimeres are at 

 right angles to this chief axis. In addition to this dominant 

 radial symmetry, there is seen even in the adult a suggestion 

 of the bilateral condition. The madreporic body generally 

 occurs in only one interradius, and a plane passing through 

 it and splitting the opposite arm divides the body into two 



