ANNULOIDA : ECHINODERMATA. 153 



centre, and striated transversely. They converge to the mouth, 

 which is superior and central in position, and is surrounded by 

 five ovarian apertures. No arms are present. 



The Blastoidea are known more familiarly under the name 

 of Pentremites, and they occur most commonly in the Car- 

 boniferous Rocks. 



CHAPTER XXII. 

 HOLOTHUROIDEA. 



ORDER HOLOTHUROIDEA. The members of this order are com- 

 monly known by the name of " sea-cucumbers," " trepangs," 

 or " beches-de-mer," and are the most highly organised of all 

 the Echinodermata. The body is elongated and vermiform, or 

 rarely slug-shaped, and is not provided with a distinct test, but 

 is enclosed in a coriaceous skin, sometimes containing scattered 

 calcareous granules or spicules. The ambulacral tube-feet, when 

 present, are usually disposed in five rows, which divide the 

 body into an equal number of longitudinal segments or lobes. 

 The mouth is surrounded by a circlet of feathery tentacles, 

 containing prolongations from the central ring of the water- 

 vascular system ; and an anus is situated at the opposite extre- 

 mity of the body. There is a long, convoluted intestine. A 

 special respiratory, or water-vascular, system is usually devel- 

 oped, in the form of a system of arborescent tubes, which ad- 

 mit water from the exterior. The larva is vermiform, and has 

 no skeleton (fig. 45). 



In the Holothuricz proper, locomotion is chiefly effected by 

 means of rows of ambulacral tube-feet, or by alternate extension 

 and contraction of the worm-like body; but in \hzSynaptidcz there 

 are no ambulacra, but only the central circular canal of the 

 ambulacral system, 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. 

 In the typical forms there are five rows of tube-feet, but these 

 may be scattered over the whole body, or may be restricted 



