470 THE ANATOMY OF INVERTEBRATED ANIMALS. 



motion, and are the ambulacral feet, suckers, or pedicels. In 

 accordance with the disposition of the ambulacral vessels, the 

 pedicels are usually disposed in five longitudinal bands, which 

 are the ambulacra. Sometimes (Psolus) the pedicels are sup- 

 pressed iu two of the five ambulacra, and the other three are 

 disposed upon a flattened surface upon which the animal 

 creeps. 



In the higher Holothurids, the intestine terminates in a 

 distinct cloaca, into which two hollow ramified organs, which 

 lie in the perivisceral cavity, open. The ramifications of one 

 of these are received between the meshes of a special plexus 

 of the pseud-haemal vessels. Water is taken into, and ex- 

 pelled out of, the cloaca and these appendages, which, doubt- 

 less, subserve an excretory function, and are commonly called 

 respiratory trees. It seems probable that the ultimate 

 branches of these organs open directly into the perivisceral 

 cavity. 1 



The Cuvierian organs are simple or branched appendages 

 of the cloaca, the function of which is unknown. The inte- 

 rior of these organs is occupied by a solid substance, some- 

 times of a viscid nature. In some Holothuridea, the anal 

 aperture is provided with a circlet of calcareous plates. 



In many of the higher Holothurids the pseud-haemal vas- 

 cular system attains a great complexity, and its branches 

 not only extend over the alimentary canal, but, as has been 

 said above, closely embrace one of the branched excretory 

 organs. 



The most aberrant form of this group at present known is 

 the genus Rhopalodina. According to Semper, the body is 

 flask-shaped, and at the narrow end of the flask are two aper- 

 tures. One of these the mouth is surrounded by ten ten- 

 tacula ; the other, which is the anal aperture, is encircled by 

 ten papillae, and by as many calcareous plates. A spacious 

 clpacal cavity, provided with excretory organs, traverses the 

 neck of the flask, and opens by the anal aperture. The erul- 

 let is surrounded by a ring of ten calcareous plates. The 

 genital duct is situated between the cloaca and the gullet. 

 Ten ambulacra diverge from the centre of the enlarged aboral 

 end of the body, and extend, like so many meridians, to near 

 the commencement of the neck of the flask. In correspond- 

 ence with each ambulacrum is a longitudinal muscular band ; 

 and it is an especial peculiarity of Rhopalodina that five of 



1 Semper, loc. tit., Heft iv., p. 133. 



