2l8 MANUAL OF ZOOLOGY. 



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 (fig. 108, $), 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 ap- 

 pearance 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 Pentre- 

 mites represent the arms of the Crinoids, anchylosed with the 

 calyx, and that the longitudinal furrows of the pseud-ambulacra 

 represent the " brachial grooves " of the Crinoids ; but they 

 are peculiar in the fact they are perforated by the apertures of 

 a number of respiratory tubes. 



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 known more familiarly under the name 

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

 iferous rocks. 



CHAPTER XXL 

 HOLOTHUROIDEA. 



ORDER HOLOTHUROIDEA. Vermiform or slug - like Echino- 

 derms, with a leathery skin, in which calcareous granules and 

 spicules are generally developed. Mouth surrounded by a circlet 

 of tentacles. Sexes mostly distinct. Larva vermiform, without 

 a skeleton. 



The members of this order are commonly 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, generally, but not always, containing calcare- 

 ous deposits in the form of scattered granules or spicules, or 

 even imbricated scales. The ambiilacral tube -feet, when 

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



