110 



INVERTEBEATE ANIMALS. 



(fig. 68). It opens into a long convoluted digestive tube, which is 

 attached to the body-walls by a delicate mesentery, and terminates 

 in a large muscular chamber or " cloaca " (fig. 69), which in turn 

 communicates with the exterior by the anal aperture. Springing 

 from the cloaca in many Holothurians are two much-branchedtubes. 



Fig. 68. — Cucumaria frondosa^ 

 showing tlie crown of feathery 

 tentacles round the mouth 

 and the rows of tube-feet. 



Fig. 69. — Jlolothuroidea. Seiui-diagrammatic 

 longitudinal section of a Holotliurian. ( 

 Tentacles ; r Calcareous ring at the base of 

 the tentacles ; p Polian vesicle ; s Sand- 

 can >il; in Alimentary canal; (j Duct of 

 the reproductive organs ; cl Cloaca ; a Anus ; 

 1 1 Respiratory tree. 



which run forward towards the anterior end of the body, and which 

 are filled with sea-water from the exterior. These tubes seemingly 

 act as respiratory organs, and are usually spoken of as the "respira- 

 tory tree " (fig. 69, 1 1). The Holothurians have a nearly world-wide 

 distribution, and are found from between tide-marks up to great 



