ANNULOIDA : ECHINODERMATA. 177 



plates, firmly united together, and arranged in five inter-ambu- 

 lacral and as many ambulacral areas. (These ambulacral areas 

 are termed by M'Coy "pseud-ambulacra," upon the belief that 

 they were not pierced for tube-feet, but that they carried a 

 double row of little jointed tentacles or arms.) The pseud- 

 ambulacra are petaloid in shape, having a deep furrow down 

 the centre, and striated transversely. They converge to the 

 summit of the calyx, and each appears to have carried a row 

 of small jointed " pinnulae " upon each side. The five pseud- 

 ambulacra, radiating from the summit of the calyx, give the 

 upper surface of the body somewhat the appearance of a flower- 

 bud ; hence the- name applied to the order (Gr. blastos, a bud ; 

 eidos, form). Upon the whole, it would seem most probable 

 that the pseud-ambulacra of the Pentremites represent the arms 

 of the Crinoids, anchylosed with r the calyx, and that the 

 longitudinal furrows of the pseud-ambulacra represent the 

 "brachial grooves" of the Crinoids. 



At the summit of the calyx are six apertures, of which one 

 is the mouth, four are ovarian, and the sixth is probably partly 

 ovarian and partly anal. 



The Blastoidea are kn6wn more familiarly under the name 

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

 ferous Rbclts. 



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, or even imbricated 

 scales. The ambulacral tube-feet, when present, are usually dis- 

 posed 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 v 

 vascular, system is usually developed, in the form of a systf ~ 



M 



