104 INVERTEBEATE ANIMALS. 



stomach, and that they are dee]jly grooved on their under surfaces 

 for the radiating vessels of the watei'-vascular system, which are 

 furtlier pnitected ly a sort of internal skeleton. Tlie upper surface 

 of the body and arms is richly furnished witli calcareous matter, in 

 the form of pjrickles, tubercles, spines, and pedicellariie, these last 

 being j)eculiaily modified s])ines. The upper surface, also, exliibits 

 the madrejioriform tubercle in the foiia of a concentrically striated 

 disc placed at the angle between two of the rays ; and also the 

 aperture of the anus, when tliis is present. The mouth is placed 

 in the centre of the lower surface, and is not furnished with 

 teeth. It leads into a pouched stomach and very short intestine, 

 which usually tenninates on the upper surface by an anal ajjer- 

 ture ; but the anus is occ;\sionaIIy wanting. From the u])per 

 end of the stomach in all the Asteroidea proceeds a series of 

 much - branched memlffanous sacs, two of which are prolonged 

 into each i-ay. These sacs are of a brown or greenish colour, and 

 are often called the " hepatic cieca," as they are sujjjiosed to corre- 

 spond with the liver. The water-vasculai' or ambvdacral system is 

 in most essential I'espects identical in structure with that of the Sea- 

 urchins, making due allowance for the different shape of the body. 

 The madrepoiic jilate leads into a short .s;ind-canal, which ojiens 

 into a circular ling round tlie gullet. The circular ring carries 

 Polian vesicle.s!, and gives off the radiating ambulacral vessels, one 

 of which is piTilonged to the extremity of each ray, running at the 

 liottom of the deep ambulacral groove on the under suiface of the 

 arms. Tlie ambulacral gToove is formed by a double row of cal- 

 careous plates (the " andjulaci'al ossicles"), and the radiating ambu- 

 lacral vessel with its tube-feet lies underneath these jilates. On the 

 fither hand, the "am|)ull;e," or little bladdei-s which distend tl.e 

 tube-feet, .are situateil ahore the ambulacral ossicles, and therefore 

 in the interior fif the arm ; and the ampuUie become coiniected with 

 their res]iective tube-feet by ])assing through .a series of pores 

 formed between successive pairs of ambulacral ossicles. The ner- 

 vous system consists of a gangliated ring surrounding the mouth and 

 sending branches along each of the arms. The re]jroductive organs, 

 like the nervous system, exhibit a ladiate condition, being arianged 

 in pairs in each ray. 



The ,Star-fi.shes are found on almost all shores, but many forms 

 are properly inhabitants of dee]) water. They differ much in the 

 general shape of the body. Tn the comnion Cross-fishes (Urasler or 

 Anteria.i) the disc is small, and is furnished with long finger-like 

 rays, which are ]>roperly fixe in number. In the Crihclla: the 

 general shape is mvich the same. In the Sun-stai>i {Stilaster) the 

 disc is large and well marked, and the rays are from twelve to fif- 



