136 MANUAL OF ZOOLOGY. 



usually arranged in definite groups. These groups are termed 

 'pectinated rhombs,' but their exact function is entirely 

 unknown. 



ORDER BLASTOIDEA. The members of this order, like those 

 of the preceding, are all extinct, and are entirely confined to 

 the Palaeozoic period. The body was fixed to the bottom of the 

 sea by means of a short jointed pedicle ; it was globular or oval 

 in shape, and composed of solid polygonal calcareous plates, 

 firmly united together, and arranged in five inter-ambulacral 

 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 ambulacral areas 

 are petaloid in shape, having a deep furrow down the 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 Hocks. 



CHAPTER XXII. 



HOL TRUE OIDEA. 



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, and 

 is not provided with a distinct test, but is enclosed in a 

 coriaceous skin, usually containing scattered calcareous gra- 

 nules 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, 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 developed, in the form of a 

 system of arborescent tubes, which admit water from the ex- 

 terior. The larva is vermiform, and has no skeleton (fig. 38). 

 In the HolothuricB proper, locomotion is effected by means 

 of rows of ambulacral tube-feet ; but in the Synaptidoe, there 



