( 475 ) 

 ANOCYSTI. 



In those species in wliich the mouth and vent are at opposite ends 

 of the axis of the animal, the body is globular or hemispherical, and di- 

 vided into ten compartments by ten avenues of pores. The avenues 

 approacli ui pairs, making five of the compartments smaller than the 

 others with which they alternate. The smaller compartments consist 

 of a double row of plates, united to each other by a zig-zag line, and 

 to the larger compartments by a straight subserrated suture. Each 

 plate is covered with tubercles, on the surface next its fellows, in the 

 same compartment, but on the surface towards the opposite side, it is 

 perforated with pores, in pairs, and arranged in oblique lines, with an 

 oral direction. The portions of the plate, with the pores, are frequent- 

 ly compound. These plates increase in size, from the mouth to the 

 middle of the body, where the avenues of the pores are at the gi'eatest 

 distance, and then decrease to the pelvis, where the pores approach, 

 and terminate in a pentagonal costal plate. In the inside of the mouth 

 there are five plates perpendicular to the margin, and perforated in the 

 middle, from which the smaller compartments take their rise. The 

 large compartments likewise consist of a double row of plates, united 

 by a zig-zag line, covered with tubercles without pores, widest in the 

 middle of the body, and terminating in the five plates of the pelvis, 

 one of which, termed the ivart, is curiously puckered, and all of them 

 have a large perforation. These plates form the ring of a circular 

 space, covered by a tough skin, with tubercles, and perfoi'ated in the 

 centre by the vent. Around the mouth there is likewise a circular 

 space, formed by a tough muricated skin. The whole body is covered 

 with an integument, more or less intermixed with muscular fibres. 

 The appendices of the skin are of three kinds. 1. The prickles are of 

 different sizes, and are seated on the convex siu"face of the tubercles, 

 which are received into their concave bases, thus forming a ball and 

 socket joint, surrounded by the integument, and put in motion by its 

 agency. These serve the purpose of defence, and assist locomotion. 

 Their structure is radiated from the centre, with distinct traces of con- 

 centric layers of growth. Mr Haidinger, in his translation of Mohs's 

 Mineralogy, vol. ii. p. 91, has stated, that, in a fossil state, " every 

 one of the spines of Echini consists of a single individual (Rhombohe- 

 dral Lime Haloide) perfectly cleavable, and the axis of which is paral- 

 lel to the axis of the spine. But, what is still more remarkable, the 

 spines of these animals possess the same property, even in recent 

 specimem of the latter, and it appears, that the carbonate of lime crys- 



