650 THE INVERTEBRATA 



In the body wall, under the dermis, which contains minute ossicles 

 of a form characteristic of the species, there are transverse muscles 

 between the radii and longitudinal muscles under the radii. The radii 

 contain the same structures as in the Echinoidea. Only one tube runs 

 from the radial water vessel to each tube foot. 



The alimentary canal (Fig. 456) is slung to the body wall by a 

 mesentery. It runs (except in Synapta) an S-shaped course, looping 

 almost the whole length of the body — backwards in the mid-dorsal 

 interradius, forwards in the left dorsal interradius, and finally back- 

 wards in the right ventral interradius to the anus. It starts as an 

 oesophagus , enclosed in a calcareous ring of ten ossicles, five radial and 

 five interradial, which has been thought to represent the auriculae 

 and lantern of an echinoid. The oesophagus is followed by a short 

 muscular region known as the stomach, this is succeeded by a thin- 

 walled intestine which forms the greater part of the canal, and finally 

 there is a short, wide cloaca. Into the latter usually open two long, 

 branched respiratory trees, whose ramifications end in thin-walled 

 ampullae through which water, when pumped in by contractions of 

 the cloaca, passes into the body cavity, carrying oxygen to the coelomic 

 fluid, and so to the organs. In //o/o^^«n« and a number of other genera 

 the lower branches of the respiratory trees are converted into Cuvierian 

 organs, tubes covered with a sticky substance which in sea water 

 elongate and form a mass of sticky threads. When the animal is at- 

 tacked or otherwise irritated a violent contraction of the muscles of 

 the body wall sets up in the perivisceral cavity a pressure which 

 ruptures the cloaca and drives out the Cuvierian organs (and often 

 subsequently the rest of the alimentary canal). The enemy is en- 

 tangled by the sticky threads. Except in the Dendrochirotae, the food 

 is extracted from sand or mud which is shovelled into the mouth by 

 the tentacles. Dendrochirotae entangle small organisms on their sticky 

 tentacles and, putting the latter one by one into the mouth, contract 

 upon them and, by drawing them out, strip ofi" the catch. 



The axial organ is represented only by a cord-like genital stolon 

 near the gonoduct. The aboral sinus and vascular ring, genital rachis, 

 and apical nervous system are absent. The lacunar system consists of 

 an oral ring, radial "vessels", "dorsal and ventral vessels" of the 

 alimentary canal, and a plexus on the latter. In the middle part of the 

 intestine of Holothuria and many other genera, the "dorsal vessel" 

 hangs from it on a perforated fold of the peritoneum, but remains 

 connected with it by a plexus known as the rete mirabile. In perfora- 

 tions between the strands of this plexus the branches of the left 

 respiratory tree are entangled. The condition of the water vascular 

 system is that described on p. 627; for that of the genital system see 

 p. 629. 



