246 ECHINODERMATA— ASTEROZOA phylum iv 



as the generative organs. Ämhulacrol feet disposed in rows along deep open grooves 

 on the under or actinal surface of the arms. 



Starfishes have typically five arms (but in some cases as many as eight, 

 teil, twenty, forty, or more), which are prolongations of the central disk, 

 usually not sharply marked off from the same. The integumentary skeleton 

 consists of plates which are either contiguous with one another along their 

 edges, overlapping or united in a reticulate fashion, and covered with a leathery 

 skin. The calcareous plates often bear movable spines, or they may be 

 tuberculated or granulated. Modified spines with a special function and 

 called pedicellariae are found in most Asteroidea and are often conspicuous. 

 They never occur in Ophiurans or Holothurians, and are not known among 

 Pelmatozoa, but what appear to be homologous organs occur in nearly 

 all Echini. The abactinal surface usually exhibits a central or sub- 

 central anus, and also a madreporite, which is situated in one (rarely two 

 or more) of the interradii. The madreporite is covered with labyrinthic 

 furrows, and is perforated for the admission of water into the so-called ston^ 

 canal, whence it is conveyed into the water-vascular ring surrounding the 

 mouth. The protrusive caecal processes (papulae), which in the more 

 primitive forms are restricted to the dorsal surface, but in the more specialised 

 are distributed over the whole body, serve as respiratory organs, the body 

 fluids being brought into close contact with the oxygenated water. 



The mouth occupies the centre of the ventral surface, and is pentagonal in 

 contour, owing to the projection of five pairs of interradially disposed oral 



plates. Each of the arms is traversed on 



^ ^ /k A the under or oral side by a broad and deep 



»^^ J|j_.W furrow, which tapers gradually in passing 



^^0 TO tii) from the mouth to the tip of the arm, where 



Fig. 348. it is termiuatcd by a simple grooved plate 



oJar'pils „f \^rTSä^i,rAt(^JS- im l<^i}^ät}^oeular plate. The roof 

 Pentago7Mster(^) irom tagonaster (?) from of each ambulacral furrow IS formed bv two 



the Upper Jura of the Upper Jura of jr r^ ti ^i i j. z 



Streitberg. 2/^. Streitberg. i/j. rows Ol raiter-like, rather elongate, ambu- 



lacral ossicles, the inner ends of which are 

 held together by muscles (Figs. 348-350). Eunning along the centre of the 

 groove on its ventral side are placed in succession the radial water -tube, 

 blood-vessel and nerve cord. These are all homologous with the like-named 

 organs of Ophiuroids. 



The form of the ambulacral ossicles difFers in diiferent genera. In all 

 Recent forms the ends are directly apposed against one another in the median 

 line of the ambulacral grooves ; but in Paleozoic forms they were apparently 

 arranged in alternate rows, and inclined towards one another at a very small 

 angle. Each pair of ambulacral plates is excavated at the sides, so as to give 



Comp. Zoölogy, Cambridge, v., 1877.— Viguier, C, Anatomie comparee du squelette des Stellerides. 

 Arch. de zool. experim., vii., 1878.— Sladen, W. P., Eeport on the Asteroidea. Scient. Kesults, 

 Challenger Expedition, 1889, vol. xxx.—Fraas, E., Die Asterien des weissen Jura. Palaeonto- 

 graphica, 1886, vol. xxxii.— Gregory, J. W., On Lindstroemaster and the Classification of the 

 Palaeasterids. Geol. Mag., 1899, dec. 4, vol. vi.—Linstow, 0. v ., Zwei Asteroiden aus mär- 

 kischem Septarienton, etc. Jahrb. k. Preuss. Landesanst., 1909, vol. xxx. pt. 2.— Schöndorf, F., 

 Die Organisation und systematische Stellung der Sphaeriten. Arch. f. Biontologie, 1906, vol. i. 

 — Idem, Paläozoische Seesterne Deutschlands. Palaeontogr. 1909-10, vols. Ivi., Ivii. — Ide7n, Die 

 fossilen Seesterne Nassaus. Jahrb. Nassauischen Ver. Naturk., Jahrg. 62, 1909.— Jludson G. ff., 

 A fossil Starfish with ambulacral covering plates. Ottawa Nat., 1912, vol. xxvi. 



