l6o MANUAL OF ZOOLOGY. 



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 Carboni- 

 ferous Rocks. 



CHAPTER XXII. 



HOLOTHUROIDEA. 



Order Holothuroidea. — The members of this order are 

 commonly known by the name of " sea-cucumbers," " tre- 

 pangs," or " beches-de-mer,'' and are the most highly organised 

 of all the Echinodermata. The body is elongated and vermi- 

 form, 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 

 extremity of the body. There is a long, convoluted intestine. 

 A special respiratory, or water-vascular, system is usually de- 

 veloped, in the form of a system of arborescent tubes, which 

 admit water from the exterior. The larva is vermiform, and 

 has no skeleton (fig. 47). At a certain period of their exist- 

 ence, the young Holothurians are barrel-shaped, with trans- 

 verse rings of cilia. They rotate rapidly on their long axis, 

 and have at this stage been described as a distinct genus 

 under the name of Auricularia. 



In the Holothuria proper, locomotion is chiefly effected by 

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

 tension and contraction of the worm-like body; but in the 

 Synaptid<z 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 givmg off branches to the tenta- 

 cu^a; 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 



