380 



oecidentalis (Salter), wlien silicified specimens are treated with acid the 

 plates are easily separable, and, therefore, although in close contact, they 

 were not anchylosed together. 



The endorhin is also composed of small rhomboidal plates arranged in 

 curving rows ; but it differs from the ectorhin in being perforated by 

 numerous small circular orifices, one of which is situated at each point 

 where the angles of four plates meet. From the centre of each of the 

 plates of this integument there radiate four small canals, one proceeding 

 straight to the middle of each of the sides of the plate, where it communi- 

 cates with a similar canal in the adjoining plates. Each one of these 

 plates is, therefore, connected by these canals with the four plates in con- 

 tact with it. The canals are excavated in the substance of the plates, and 

 communicate with the central canal of the transverse tubes. The canals 

 are not always perfectly circular, but are often flattened or irregularly 

 circular. The endorhin varies greatly in the extent to which it is deve- 

 loped. In some specimens the plates are well-defined and rhomboidal, 

 with perfectly circular pores at the angles. In others the plates are not 

 at all defined, the ectorhin being one continuous integument without 

 sutures, but always with the full complement of pores. The latter in 

 such specimens are not all circular, but are variously shaped orifices 

 sometimes with rough edges. There are also specimens in which the 

 endorhin consists of only a thin film, capping, as it were, the tubes and 

 inclosing the canals, the pores being proportionally larger than they are 

 in those with weU-developed j)lates. The end of each tube, in these spe- 

 cimens, forms an irregular, rounded tubercle instead of a rhomboidal 

 plate. 



The tubular skeleton above alluded to consists of numerous small, 

 straight, rarely curved, cylindrical tubes or hollow spicula, placed parallel 

 to each other and at right angles to the planes of the body-wall of which 

 they form the greater portion. They connect, and at the same time keep 

 asunder, the ectorhin and the endorhin. One of these tubes springs from 

 the centre of each plate of the ectorhin : it is, at its base, or next to the 

 ectorhin, very slender, but enlarges so as to attain its full thickness at 

 about one-fourth of its length, and then remains at the same diameter 

 throughout until it reaches the endorhin, by a single plate of which its 

 inner extremity is, as it were, capped. The outer extremity of each tube 

 has four small slender stolons, one proceeding to each of the four angles 

 of that particular plate of the ectorhin from the centre of which it (the 

 tube) springs. It there seems to form a connection with the stolons of 

 the three adjacent plates whose angles meet at that point. The stolons 

 are so arranged that one of them always points inwards towards the 

 nucleus, and another on the opposite side of the tube outwards or upwards. 



