384 COMPARATIVE ANATOMY CHAP. 



The bundles of hydrospire tubes or pouches of the Blastoidea 

 have been compared with the " bursae " of the Ophiuroidea. They are 

 said to have served, like these latter, for respiration and for the 

 ejection of the genital products. The similarity of these apertures is 

 specially marked if we compare Orophocrinus with an Ophiurid. 



(b) The Stem. 



With the exception of the genera Pentephyllum, Eleutherocrinus, 

 and Astrovrinus, which, at least in the case of all known adults, were 

 non-pedunculate, the Blastoidea were attached to the substratum by 

 a jointed stem without cirri (Fig. 264, p. 314). 



VII. Cystidea. 



The study of the skeleton of this ancient class, which is limited to 

 the palaeozoic age, has no very great comparative interest. The organi- 

 sation of the very heterogeneous groups which are classed together 

 under this heading can only to a small extent be understood from 

 their skeletal remains. According to the structure of the skeleton, we 

 can perhaps distinguish two principal groups : the Cystovriwidea, whose 

 skeleton consists of comparatively few definitely arranged plates, and 

 which in some forms approximate the Crinoidea ; and the Eucystidea, 

 whose skeleton consists of a very large number of plates, showing no 

 definite recognisable order. 



It is characteristic of most Cystidea, that all or some of the 

 plates of the skeleton are perforated in various ways by pores, which, 

 however, never seem to establish communication between the interior of 

 the calyx and the exterior. It is difficult to ascertain the significance 

 of these pores. They could not serve for the passage of the ambu- 

 lacral feet, since the pore canal, as already said, does not stand in 

 direct communication with the interior of the calyx. It is now pretty 

 generally accepted that, as the water passed through them, they served 

 for respiration. The following principal forms of pores can be 

 distinguished : 



1. Scattered single pores. 



2. Scattered double pores (the pores being united in pairs) (Fig. 

 260, p. 312). 



3. Double pores arranged in rhombs. In this case, the two pores 

 of a double pore are found on two neighbouring plates, and are con- 

 nected by a furrow or a canal, which sometimes runs at the outer 

 and sometimes at the inner side of the plate. This canal or furrow 

 lies at right angles to the suture between the two plates, and 

 the suture itself lies diagonally to the rhomb formed by the pores. 

 Such pore -rhombs may occur on all the plates of the Cystid 

 test, or again may occur singly. In the latter case, the two halves 



