434 . COMPARATIVE ANATOMY chap. 



stirring up tlie sand. On other parts of the ambulacra, slender tentacles without 

 prehensile discs occur, to which a tactile function has been ascribed. Still more 

 interesting are the ambulacral appendages of the anterior unpaired ambulacrum, 

 which are certainly to a still higher degree tactile organs. These vary in shajie ; in 

 all young Spatangoida and many adults they are distinguished by their remarkable 

 size, and help to emphasise the bilateral symmetry of tlie whole bod}^ In Sjiatangiis 

 and other genera they end in a flat disc, the edge of which is drawn out into short, 

 solid, knobbed processes, which are supported by calcareous rods. The whole 

 terminal disc thus looks like a beautiful rosette. 



As compared with the ordinary tube-feet, truly gigantic proportions are attained 

 in the genera Accste and ^^rope by the ambulacral appendages, which lie in the 

 depressed anterior ambulacrum within the pcripetaloid fasciole. They are found 

 only in small numbers, yet in their contracted condition they almost completely fill 

 the depression from whose base they rise. Tlreir ends are provided with large discs. 



Turning to the finer structure of the ambulacral ajtpendages of the Echinoidea, 

 their wall is found to consist of the typical layers. In the locomotory tube-feet of 

 the regular Echinoids, the inner layer of the connective tissue is specially modified 

 as an elastic membrane, with circular fibres. Calcareous corpuscles are wanting 

 only in the respiratory tentacles of the apical surface of the body. In all other parts 

 of the body they are found in great numbers in the stalk, while in the terminal discs 

 of the tube-feet they take the form of delicate, circular, terminal plates usually com- 

 posed of several pieces. The whole surface of the tentacles, except that of the terminal 

 discs, is ciliated. A nerve enters each tube-foot, running in the epithelium until 

 near the tip, where it forms a lateral ganglion, which can be externally recognised as 

 a swelling. From this ganglion, the terminal ajjparatus of the tulie-foot is innervated. 

 In tube-feet with terminal discs, the epithelium at the edge of the disc is differen- 

 tiated as a deep sensory epithelium, and witlnn it runs a basal nerve ring, connected 

 with the lateral ganglion by two nerves. Where the tentacles end in knobs (tactile 

 tentacles and sessile knobs of the Clypeastroida) or carry knobbed processes on their 

 terminal discs (ambulacral brushes, rosette-like tube-feet of the anterior unpaired 

 ambulacrum of the Spatangoida) these knobs are caused by the great thickening of 

 the sensory epithelium. In the ambulacral gills of the Clypeastrida: {Echinncyamvs, 

 Echinodiscus) the epithelium occasionall}" thickens to form sensory papillae. The 

 sensory epithelia appear everywhere to carry stiff' sensory setiie or hairs. 



The lumen of the tube-feet at its central and basal parts is not infrequently 

 traversed by transverse muscle fibres ; occasionally it appears to be double — a septum 

 consisting of transverse bands continues for some distance into the tube-foot, the 

 partition in the test between the two apertures of the double pore. The cavitj'^ of the 

 large terminal discs of the paint-brush tentacles in the Spatungoida is traversed by 

 concentric, much-perforated septa (Fig. 366). The ambulacral gills and their ampullae 

 are traversed by bands arranged radially around an axis, in such a way that the 

 fluid contained is forced to circulate at the iieriphery. 



3. Asteroidea. — The ambulacral appendages always take the form 

 of tube-feet, and stand in two or four longitudinal rows in the ambu- 

 lacral furrows which run from the mouth to the tips of the arms. It 

 has already been pointed out (p. 353) that tlie tube-feet, even when 

 apparently present in four rows, in reality belong only to two. That 

 there are only two rows is verj^ clear in young animals. In young 

 Asteroids all the tube-feet are alike ; all have conical ends, with 

 rounded tips. This is still the case in many adult Asteroids {Astro- 



