AXATOMY OF THE ECHINODERMS. 261 



the latter into ganglia, the minute structure of the oral ring being identical with that of its radial 

 extensions. But although anatomical investigation fails to reveal the presence of a brain, or even of a 

 ganglion, yet the result of physiological research is to indicate that the oral nervous ring is the seat of 

 a centralising influence, which proceeds outwards from it, and regulates the movements of the tube- 

 feet organs of which more or less use is made in locomotion. 



Immediately within the oral nervous ring is an annular blood-vessel (Figs. 10, 12, 20, ob), from 

 which radiating trunks extend (Figs. 9, 10, 12, 13, 21, b). It is connected with a more or less 

 complicated network of vessels, which surrounds the digestive apparatus (Fig. 20, ib). In most 

 Echinozoa* it also communicates, by means of a bundle of vessels that run nearly in the vertical axis 

 of the body (Fig. 10, cp), with an aboral blood-vascular ring (Figs. 10, 12, ab), from which vessels 

 (Fig. 10, gv) proceed to the generative organs. This vascular bundle (Fig. 10, cp) was formerly 

 regarded as a heart, and has been described as performing rhythmical contractions. This, however, is 

 very doubtful ; while some authors go so far as to say that the organ in question is mereiy a gland 

 with an excretory duct, which opens upon the aboral surface of the body, and is unconnected with 

 any portion of the vascular system.f 



The special feature in the anatomy of the Echinoderms is a set of tubes which communicate with 

 the exterior, ana serve the purposes both of respiration, and of locomotion. It is known as the water- 

 vascular system, and consists of an oral ring (Figs. 12, 20, wr) and radial extensions (Figs. 9, 12, 13, 

 20, u-r), like those of the blood- vascular system, which lies immediately external to it. The radial 

 vessels give otf numerous lateral branches, which enter contractile processes of the body-wall, the tube- 

 feet, or tentacles. In the Crinoids, which lie on their backs, with their mouths upwards, the tentacles 

 are exclusively respiratory in function. But most of the Echinozoa live mouth downwards, and the 

 tube-feet are used in locomotion. They are, inconsequence, often spoken of as "the ambulacral + 

 feet," while the whole system of water- vessels is called the ambulacral system. In most Echinozoa 

 its communication with the exterior is effected by a tube which starts from the water-vascular ring, 

 and opens on the surface of the body by a sieve-like plate, the " madreporite " (Fig. 1, m). This 

 water-tube, as we will call it, is sometimes known as the sand-canal, or stone-canal, on account of the 

 limestone deposits in its walls. It lies close to the central plexus, and is bound up together with it 

 in the same membranous sheath. 



In most Holothurians and in the Crinoids there are one or more tubes depending from the 

 water-vascular ring, and opening into the body-cavity, which communicates directly with the exterior 

 (Fig. 20, wf); and as far as can be judged from the conflicting statements of different natumlists, these 

 are the only Echinoderms in which there is a direct communication between the body-cavity and the 

 exterior. Protoplasmic corpuscles of different kinds are dispersed in the fluid which it contains. 

 Some resemble the white corpuscles of the blood of a vertebrate animal, while others exhibit more 

 active changes of form, and put out long thread-like extensions of their substance. The body-cavity 

 of an Urchin or Holothurian also contains granular masses, which are coloured with a brown substance 

 that contains iron, and changes its tint under atmospheric influences, so that it is probably connected 

 with the process of breathing. This function, however, is mainly performed by the water- vascular 

 system. It contains minute red corpuscles, tinged with haemoglobin, the oxygen-carrying material 

 that colours the corpuscles of our own blood, and has also been detected in that of Molluscs, Crustacea, 

 Insects, and Worms. The cilia lining the water-vessels keep up continual currents in their interior, 

 and the circulation of the corpuscles which is due to these currents is increased by the contraction of 

 the walls of the water-vessels, and by the continual motion of the tube-feet or tentacles, whether they 

 be used for locomotion or not. 



The sexes are distinct in most Echinoderms, and the fertilised ova generally pass through a 

 complicated process of metamorphosis before assuming their adult form. They are hatched as 

 uniformly ciliated free-swimming embryos, which gradually acquire a digestive tube with two 

 openings. The cilia become restricted to one or more transverse ridges (Fig. 7), and the larva passes 



* Greek, echinus, hedgehog ; zoon, animal. 



t These statements are probably erroneous. They are based almost entirely upon the results of injections, which are 

 much less likely to give accurate results than the study of continuous series of thin sections through any organ and the 

 structures connected with it. 



J Latin, ambulacrum, a place for walking. Etym., pierced with small holes, like the coral known as madrepore. 



