THE STARFISH. 



265 



Opium-ids, which commence with the same period. The Urchins are represented in the Lower Silurian 

 by a single aberrant form, but more appear in the later Palaeozoic beds ; while the Mesozoic and 

 Tertiary rocks contain a great variety of types. Fossil remains of the soft-bodied Holothurians are 

 naturally rare, but they can be traced as far back as the Carboniferous period. 



The Echinoderms fall into two very natural groups: viz., (1) the Eckinozoa, including the 

 Urchins, Stellerids, * and Holothurians, all of which crawl about by the aid of their tube-feet, with 

 the mouth downwards or at one end of the elongated body ; and (2) the Pelmatozoarf or Stalked 

 Echinoderms. In the latter group the dorsal region of the body is pro- 

 duced into a stalk, by which the animal fixes itself with its oral surface 

 upwards (Figs. 8, 18). In the Feather-stars, which form the majority of 

 recent Criiioids, the stalked condition is a temporary one (Fig. 8), the 

 body eventually detaching itself from the larval stem, and settling down on 

 its own account, though still in the same relative position, i.e., with its 

 oral surface upwards (Fig. 19). Nearly all the fossil Criiioids were 

 stalked, as were also the extinct Blastoidea and Cystoidea, though a few 

 sessile forms are known. 



Among the Echitwzoa, the Stellerids are those of which the anatomy 

 is most completely known ; and as the members of the two classes to which 

 this name is applied resemble one another in very many respects, it will 

 be convenient to take them as the starting-point of our investigations 

 into Echinoderm structure. 



(1) Asteroidea.* The body of a Starfish is usually somewhat flattened, 

 and either pentagonal in outline or more or less stellate, in which case it is 

 said to consist of a central disc extended into five or more arms (Fig. 1). 

 Its shape is maintained by an internal skeleton of limestone joints (Fig. 

 9, ao). This is covered, though not closely, by a tough leathery skin, in 

 which are embedded granules and plates of limestone, many of them 

 bearing spines. Some of the spines, which are known as paxillae, assume 

 the form of a stem with an expanded brush-like end (Fig. 9, pax). The 

 mouth occupies the centre of the under surface of the body, and a deep 

 groove, the " ambulacral groove," proceeds from it along each of the arms 

 (Fig. 9, ay}. This groove is nearly filled with the tube-feet, or tentacles 

 (Fig. 9, t), which are connected with the ambulacral or water-vessel, 

 situated in the middle line of the arm (Fig. 9, w), and are largely used 

 in locomotion. Appended to each of the lateral branches of the water- 

 vessel that proceed to the tube-feet is a minute muscular water-sac, or F - s. PENTACRIXOID LAKVA 

 ampulla, by the contraction of which water is driven into the tube-foot OF THE ROSY FEATHEH-STAH 

 so as to expand it. The tube-feet themselves are also contractile, and when (Comatula rosacea). 



. . , , . . i , A, quite young, before the opening ot 



several of them which are attached to any object by their terminal suckers the. cup and the appearance of the 



' radial plates: u, nearly mature with 



are made to contract, the result is that the body is slowly drawn towards %%& t J SHaRrffriSaJSiP' 

 the fixed point. Other tube-feet are then distended and projected forwards, 



to take fresh hold farther on, while those previously fixed are detached by water entering them from 

 the ampullae, and so the movement goes on. The radial water-vessels all communicate with an oral 

 ring provided with water-sacs, the " Polian vesicles," || which are similar to those in the arms, but do 

 the same work on a larger scale. They are attached to the water-vascular ring between the origins of 

 the radial trunks ; and the single water-tube which communicates with the exterior by the madreporic 

 plate (Fig. 1, m) occupies a similar interradial position. It is enclosed in a common sheath with the 

 central plexus of the blood- vascular system (Fig. 10, cp), which unites the oral blood- vascular ring (ob) 

 with the aboral ring (ab), connecting the ten genital and the two gastric vessels (gv; pb). Radial 

 wanks (Figs. 9 3 10, b) proceed outwards from the oral ring beneath the water-vessels, and send minute 



v Latin, stella, a star ; Greek, eidos, form. X Greek, aster, a star ; eidos, form, 



f Greek, pelma, a stalk ; soon, animal. Latin, ampulla, a flask. 



|| Named after Poli, the anatomist who discovered them. 

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