COMPARATIVE ANATOMY 



CHAP. 



382 



radial apertures, the so-called spiracles. Each of these apertures 

 leads into the hydrospire pouches in such a way that the halves of 

 two adjoining ambulacral areas have one common spiracle for 1 

 hydrospire pouches. 



Each spiracle forms a depression in the central part of the corresponding deltoid 

 plate and is further bordered by the proximal side plates, and by the proximal 

 ends 'of the lancet plates. Occasionally, each spiracle is more or less distinctly 

 divided into two by a vertical median ridge (septum) projecting into it 

 deltoid plate. In the posterior interradius, the spiracle is confluent with the 

 anus. 



The hydrospires (Fig. 332) are calcareous pouches or tubes 

 united in groups. 



A group of such calcareous pouches is arranged symmetrically on each side of 

 the middle line of, and in close connection with, each ambulacrum. The pouches, 



2, 3 



FIG. 33--'. Section through an ambulacrum of Pentremites, diagrammatic. 1, Deltoid, while 

 lower down is the radial ; ~2, hydrospire pore ; 3. accessory side plate ; 4, side plate ; 5, lancet plate, 

 with its ambulacral canal 7 ; 6, covering plates ; 8, common channel, into which the hydrospire 

 pouches (11) enter at 9 ; 10, base of a phmula ; 11, hydrospire pouches ; 12, lower lancet plate. 



which hang down into the cavity of the calyx, are parallel to one another, and 

 stretch from the distal end of the ambulacrum to its proximal end, as far as to the 

 spinu'lo, through which they open externally. In addition to this opening, each 

 hydrospire pouch possesses a slit-like aperture extending along its whole length, in 

 the ambulacral area. These hydrospire folds are hidden, lying partly under the side 

 plates and partly under the lancet plate. After the removal of these latter pieces, 

 at we have seen, they appear at the surface. They vary in number from three to 

 nine. The concealed hydrospire canal (8), into which, on each side, the hydrospire 

 ] touches open through their clefts, communicates with the exterior by means of the 

 hydrospire pores already mentioned. 



The hydrospire pouches or tubes have thus a double manner of communication 

 with the exterior, viz. through the five or ten spiracles round the mouth, and 

 through the numerous hydrospire pores at the lateral edges of the ambulacra. 



In certain species, the peristome was overarched by a roof of covering plates, 

 f.,i th.- most part irregularly arranged (Fig. 331, 1) ; at their centre, five oral plates 



