268 



NATURAL HISTORY. 



the arm-bases, that dovetail, as it were, into the disc, and are visible on its under side, separated 

 from one another by groups of regularly-arranged plates, which converge towards the central mouth. 

 Each arm-base is separated from the plated interradial areas at its sides by slit-like openings, which 

 are usually single, but occasionally double. These are known as the genital slits, and lead into thin 



walled pouches at the sides of the rays, 

 to which a two-fold function has been 

 assigned. In a living Ophiurid a 

 double current of entrance and exit is 

 visible around these genital slits, its 

 cause appearing to lie in the alternate 

 expansion and contraction of the disc ; 

 and the pouches thus seem to serve as 

 a kind of internal gills, or breathing 

 apparatus. The water which enters 



them brings in oxygen, which it eX- 

 12. DIAGRAM ON A LONGITUDINAL SECTION THROUGH THE DISC . . . - , 



AND AN ARM-BASE OF AN OPHIURID. (After H. Ludwig.) changes for carbonic acid with the 



V, mouth; I,, lip; o, stomach; c, body cavity; ca, its extension into the arm; et, con- Water in the body-Cavity through the 

 nective tissue bands; AI, A2, A6, successive ambulacral ossicles; 81,82, &c., lower arm- 

 plates; oa, oral anj,'le: j)a, pale* angulares; ta, torus anprularis; m, muscle; ms, mouth- tliin wall rvf flip nnilfh find tllPTl (TOPS 

 shield: 06, aboral blood-vascular ring; ob, oral blood-vascular ring; b. radial blood-vessel; CU ' cU1U 

 w, radial water-vessel; wr, water- vascular ring; v, Polian vesicle; bf, buccal tentacles; ,.+ v> T7 x^ i-o+iii-n nnvrpnt TViP 

 nr, oral nervous ring ; n, radial nerve. OUU oy tne Kiu J.I1 



ovaries of the Ophiurids open into 



these pouches, and the ripe ova may either be carried out through the genital slits by the efferent 

 currents, so as to undergo their larval metamorphoses independently of their parent, or they may 

 remain within the pouches, and undergo a direct and more rapid development, as has been mentioned 

 above. 



At the inner angle of each interradial area on the under surface of the disc is a plate known as 

 the " mouth-shield " (Fig. 12, ms). Between each of these and the mouth is a complicated arrange- 

 ment of plates, constituting what is called an oral angle (Fig. 12, oa, ta, &c). At the apex of this 

 are a number of short flat processes, the palece angulares (Fig. 12 'pa), while its sides bear numerous 

 smaller processes, the " mouth-papillae." These serve as strainers, keeping foreign bodies that are 

 not wanted for food from entering the stomach. The palese angulares probably serve much the same 

 purpose, though they are often spoken of as teeth. They have, however, little or no crushing power, 

 as there is usually hardly any room for any play of the oral angles to and from the central axis of the 

 body. The mouth of an Ophiurid is surrounded by twenty tentacles, two on either side of each oral 

 angle, which is pierced for their passage (Fig. 12, bf). These buccal tentacles, which are merely 

 the modified tube-feet of the two first arm-joints, are in a state of continual movement. They assist 

 the food in entering, and they also serve to clear away the undigested residue, which is ejected from 

 the mouth, as there is no second opening to the stomach. This organ (Fig. 12, G) is a wide-mouthed 

 bag, attached to the sides and top of the disc by bands of connective tissue (ct), and capable of a 

 certain amount of protrusion ; but there are no extensions of this simple digestive apparatus into the 

 arms, as there are in the Starfishes. 



The plates making up the oral angles are rather thick, as compared with the height of the disc, 

 and the water- vascular ring lies in a groove on their upper surface (Fig. 1 2, wr). It communicates by 

 a short water-tube with pores in one of the interradial mouth-shields, which represents the madreporite 

 of the Starfish. Four Polian vesicles may also be connected with it (Fig. 12, p), one for each of the 

 remaining interradii ; but there are sometimes none at all, while on the other hand they may take the 

 form of numerous irregular blind tubes. 



The blood-vessels and nerves have the same relation to the water- vascular system as in the Starfish. 



The central plexus connecting the oral and the aboral blood- vascular rings is enclosed in a common 

 sheath together with the water-tube, just as in the Starfish. The aboral ring (Fig. 12, ab) lies 

 immediately beneath the radial shields at the base of each ray ; but it dips down in the interradial 

 spaces alongside the genital slits, and rests on the mouth-shields, one of which is perforated by the 

 water-pores. Consequently the central plexus and the water-tube descend from the oral ring 

 instead of ascending, as they do in the Asterids. 



