CHAPTEE XIX 



ECHINODERMATA {CONTINUSD) : HOLOTHUEOIDEA = SEA-CUCUMBERS 



CLASS IV. HOLOTHUEOIDEA 



This class of the Eleutherozoa comprises those sausage-shaped, 

 leathery Echinodermata familiarly known as Sea -cucumbers. 

 They are named Holothuroidea from oXodovpiov, an animal 

 described by Aristotle, and believed to belong to this class. 



The Holothuroidea resemble Echinoidea in the fact that the 

 radial canals of the water-vascular system run backwards and 

 upwards from the ring -canal over the surface of the body, 

 terminating in small papillae near the anus, which, as in the 

 Echinoidea Endocyclica, is situated at the upper pole of the 

 body. There are, of course, no arms ; and a further resemblance 

 to Echinoidea is shown by the fact that the ambulacral grooves 

 are represented by closed epineural canals, and that the ecto- 

 derm consists of long, slender, flagellated cells interspersed with 

 gland-cells, underneath which is a plexus consisting of nerve- 

 fibres and small bi-polar ganglion cells. There are, however, no 

 spines or pedicellariae ; and Holothuroidea differ not only from 

 Echinoidea, but from all other Echinodermata, in the vestigial 

 character of their skeleton, which consists merely of isolated 

 nodules of calcium carbonate embedded in the skin. The body- 

 wall is provided with transverse muscles running across the 

 interradii, and also with powerful longitudinal muscles, running 

 along the radii, by means of which worm-like contractions are 

 carried out. Similar muscles, though much less developed, occur 

 in the Echinothuriidae, and must have been present in many 

 extinct Echinoidea in which the plates of the corona overlapped ; 

 and hence it is exceedingly probable that from some of these 



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