460 



PELMATOZOA. 



In Pentremites and in most other genera of Blastoids, the side- 

 plates are so shaped as to leave along their outer ends a series of 

 apertures or "marginal pores" (fig. 329, J>), by means of which 

 water is admitted to the " hydrospires." In many cases, however, 

 the marginal pores are not merely spaces between the attenuated 

 outer ends of the side-plates, but they are in part formed by a series 

 of still smaller ossicles, which are known as the " outer side-plates " 

 (the " supplemental pore-plates " of Roemer). In Pentremites the 

 side-plates are marginal, and leave the whole upper surface of the 

 lancet-plate exposed to view ; but in other cases they encroach upon 



Fig. 329. — a, Upper portion of one of the ambulacra of Pentremites sulcatus, enlarged, 

 showing the "covering-plates" (c) of the ambulacral groove, passing superiorly into the plated 

 canopy (m) which conceals the mouth. B, Ambulacrum of Pentremites iyri/ormis, from which 

 the covering-plates have been removed, showing the ambulacral groove (ag), the lancet-plate (/), 

 the side-plates (s) with their marginal pores, and the spiracles (s/>) at the upper end. c, Am- 

 bulacrum of the same species, from which the lancet-plate and most of the side-plates have 

 been removed, showing the "under lancet-plate" («/), the tops of the hydrospires, (A), and the 

 ambulacral opening. (After R. Etheridge, jun., and P. H. Carpenter.) 



the lancet-plate, and leave visible little more than the central food- 

 groove. 



The lancet-plate is pierced by a canal which lodged the radial 

 water-vessel, and which opens into a circular ambulacral canal sur- 

 rounding the mouth. The removal of the lancet-plate allows the 

 top of the " hydrospires " to be seen (fig. 329, c, h). In the genus 

 Pentremites, however, and in some other types, there is situated 

 beneath the lancet-plate, in the middle line of the ambulacral field, 

 an elongated, trough-like plate, which Messrs Etheridge and Car- 

 penter have termed the " under lancet-plate" (fig. 329, ul). 



True " arms," comparable with the structures so called in the 

 Crinoids, are not developed in the Blastoids. Along the outer edge 



