ANNULOIDA: ECHINODERMATA. 137 



are no ambulacra, and the animal moves by means of anchor- 

 shaped spicula, which are scattered in the integument. When 

 developed, the ambulacral system consists of a 'circular canal ' 

 surrounding the mouth, bearing one or more ' Polian vesicles/ 

 and giving off branches to the tentacula ; and of five ' radiating 

 canals ' which run down the interspaces between the great 

 longitudinal muscles. These radiating canals give off the 

 tube-feet and their secondary vesicles, just as in the Echinus. 

 There is also a 'sand-canal,' which arises from the circular 

 canal, and is terminated by a madreporiform tubercle ; but 

 this, instead of opening on the exterior, hangs down freely in 

 the perivisceral cavity. 



Fig. 38. Cucumaria communis. (After Forbes.) 



The mouth in Holothuria is situated anteriorly, and is sur- 

 rounded by a beautiful fringe of branched, retractile tentacles, 

 which arise from a ring of calcareous plates. The mouth 

 opens into a pharynx, which conducts to a stomach. The 

 intestine is long and convoluted, and opens into a terminal 

 dilatation, termed the ' cloaca,' which serves both as an anus 

 and as an aperture for the admission of sea-water to the 

 respiratory tubes. From the ' cloaca ' arise two branched and 

 arborescent tubes, the terminations of which are caacal. These 

 run up towards the anterior extremity of the body, and together 

 constitute the so-called 'respiratory tree.' They are highly 

 contractile, and they perform the function of respiratory 

 organs, sea-water being admitted to them from the cloaca. 

 The nervous system consists of a cord, surrounding the gullet, 

 and giving off five branches, which run alongside of the ra- 

 diating ambulacral canals. The generative organs are in the 

 form of long, ramified, caeca! tubes, which open externally by 

 a common aperture, situated near the mouth. There is, thus, 

 no trace of that radial symmetry which is observed in the 

 arrangement of the reproductive organs in the other orders of 

 the Echinodermata. 



The skin in the Holothurice is highly contractile, and the 



