658 



ECHINODERMA -CLASS OPHIUROWEA. 



the bait of fishermen, whence they are regarded by that class with a holy 

 horror, and the croas-fish sometimes is called by them "Devil's fingers." 

 To their credit it must be said that they serve as the scavengers of the sea, 

 and that they make excellent manure. 



OPHiuuoiriEA (Brittle-stars). 

 The brittle-stars and sand-stars, which may often be fmind hiding under the 

 rocks, or in the sea-weed, or in pools at low tide, resemble the ordinary stir- 

 fish in having five distinct arms. These, however, as shown in Fig. 

 5, are long and serpent-like, and are attached to a relatively small body 

 or disc (D, in Fig. 5). There are other differences, as may be seen 

 by a comparison of the section shown in Fig. 4 with that of the star- 

 fish in Fig. 3. Here it is seen that the digestive and generative systems 

 do not extend to the arms but are confined to the body. The arms 

 are cylindrical and have no groove ou the under side, such as exists 

 in. star-fish, but they have little openings through which the tube-feet 

 P pass. In this class, however, it is the arms that are used for locomotion 

 and not the tube-feet, so that the latter have no terminal suckers. The 

 greater part of each arm is formed by a central axis of successive calcareous 

 ossicles not unlike the vertebrae of a backbone. Each arm-ossicle (AO) is 

 composed of two parts — one on either side and unittd in the middle line. 

 The successive ossicles are connected by pairs of straight, muscular bundles, 

 and articulate with one another by tenon-and-mortise joints, according to 

 whose degree of development the arms vary in their power of coiling. The 

 arm-ossicles are encased in the tough outer skin of the arm, in which are 

 developed plates (U, upper, and S, side — arm-plates in Fig. 4). Spines (Sp) 

 are borne by these plates, and these are clearly shown in Fig. 5 (S) ; they 

 aid the animal in locomotion. The integument of 

 the disc, as shown in Figs. 5 and 6, also bears 

 small plates which are often covered with minute 

 spines. Returning to the section of the arm, we 

 see on its under side between the tube-feet, the 

 nerve, blood-vessel, and water-vessel, which give 

 oflf branches just as in the star-fish, the difference 

 being that they are here enclosed by the integu- 

 ment of the arm. The mouth is shown in Fig. 6 

 on the under surface of the disc. Around the 

 mouth are a number of short, flat processes, the 

 mouth papillae, which serve as strainers. Inside 

 the mouth are seen the five tooth-plates. 



Ophiuroids are found in all seas, usually occur- 

 ring in quantities. They are most abundant in 

 the rock-pools of the tropics. They gather their 

 food to them by sweeping their arms over the 

 sea-bottom, and thus conveying any suitable substances to the mouth. 

 Figs. 5 and 6 represent a daisy brittle-star, OpMophoJis aculeata, which 

 occurs all round the British coast. The colloquial name is due to the 

 resemblance to a daisy caused by the intermingling of spines and plates on 

 the surface of the disc. The popular name, brittle-star, applied to the 

 ophiuroids is due to their fragile nature and their habit, so distressing to 

 the collector, of breaking themselves into small portions when touched. 



F:g. 6. — Under sttrfack of Ophi- 

 UBID HoBY (Ophiiiphotis acu- 

 leata). Natural eize. 



