266 



NATURAL HISTORY. 



branches to the successive tube-feet (Fig. 10, bf). External to the blood-vessels are the radial nerves 

 (Fig. 9, n), communicating with an oi-al nervous ring, and sending off very minute tentacular branches. 

 Each nerve terminates at the bent-up extremity of the arm in a pigmented spot, containing clear 



lens-like bodies, and serving as an eye. Close 



Tlj\ JVJj. \\J\//j fv. to it is the terminal tentacle of the arm, which 



has no sucker, but is excessively sensitive, and 

 appears to be a very delicate organ of touch. 



The mouth leads by a short gullet into a 

 wide stomach, the lower part of which is pro- 

 duced in the direction of the rays into five 

 large sacs with folded walls. Above the origins 

 of these sacs the stomach suddenly narrows, and 

 then enlarges into a pentagonal cavity, from the 

 angles of which five forked tubes extend into 

 the rays. Each fork is the stem of a long tree- 

 like mass, which is formed of dense branches of 

 from four to six pear-shaped follicles, all con- 

 nected with the central stem. These pyloric 



Fig. 9. DIAGRAMMATIC REPRESENTATION OF A CROSS-SECTION n J /T?- n \ 



OF AN ARM OF THE COMMON CROSS-FISH (Atenu rul*n). C8eCa > aS th6V ^ Called (* 9 ' **>' a SU P" 



On the left side the section is supposed to pass between two of the ambulacral posed to represent the liver of the higher animals. 

 ossicles, but on the right side through one of them (oo) ; ag, ambulacral :_. , . , . , , , , 1 



minates in a mimite anal pore, situated near the 

 centre of the aboral face of the body. Breathing is carried on partly 

 by the tentacles of the water-vascular system (Fig. 9, t), and partly 

 by thin-walled tubular processes of the external skin (Fig. 9, br), 

 which are ciliated internally, and are in direct communication with 

 the body-cavity ; so that a free interchange of gases can take place 

 between the water which they contain and that which bathes their 

 external surface. 



The paired genital glands are situated interradially at the junction 

 of the body with the arms, into which they extend for a greater or 

 less distance (Fig. 9, ov). Each gland is divided into a number of 

 berry-like clusters, which communicate with the exterior by one or 

 more genital pores. These are either situated in the angles between 

 the arms, or, in the case of the more elongated glands, upon the arms 

 themselves (Fig. 9, gp}. 



The internal skeleton of each arm consists of two longitudinal 

 series of plate-like joints, the "ambulacral ossicles" (Fig. 9, no), 

 which lean against each other in the middle line above, so as to form 

 the sides and roof of the ambulacral groove (ay). Between each 

 ossicle and those in front and behind it are small pores, produced by 

 the fitting together of notches upon the front and back faces of the 

 successive joints. The branches from the radial water-vessels (w) to 

 the tube-feet (t) pass outwards through these pores. The lower 

 ends of the ossicles abut against a series of short and thick 

 " adambulacral plates" (Fig. 9, ap\ which form the edges of the 

 groove, and usually bear spines (sp~) ; while the sides of the arms are 

 protected by a variable number of lateral or marginal plates, also 

 bearing spines (Fig. 1). In some cases also there is an external skeleton of well-defined plates on the 

 upper surface of the anns, but there is generally only a mere network, more or less regularly ai-ranged, 

 and bearing clustered spinelets, or paxillse (Fig. 9, pax). 



Attached to some of the larger spines, and in the intervals between them, are numerous little 



Fig. 10. DIAGRAM OF THE BLOOD- 

 VASCULAR SYSTEM OF A STARFISH. 



(After H. Ludwig.) 



cp, central plexus: x, its dorsal extremity: 

 ab, aboral blood-vascular ring ; gv, genital 

 vessels ; pb, gastric vessels ; ob, oral blood- 

 vascular ring; b, radial blood-vessel ; bf, its 

 tentacular branches. 



