ENDOSKELETON. ANIMALS. 11 



matter disposed in ill-defined layers. The spioules differ 

 from those of the crust, and are mostly fused in groups of 

 two. PI. V. fig. 2. 



& 



A. 33. Red Coral (Corallium rubrum~). One piece is still in- 

 vested by the crust, from the other the crust has been 

 removed to show the axis, grooved on its surface for the 

 coslenteric canals of the crust. A drawing shows the 



o 



structure of the axis, which is composed of layers of mixed 

 calcium carbonate and organic matter ; in it are imbedded 

 numerous spicules derived from the crust. PL VI. fig. 1. 

 Lacaze-Duthiers, Hist. Nat. du Corail, 1864, p. 102. 



A. 34. Caligorgia verticillata, with drawings of its structure. 

 The axis consists of an intimate mixture of calcium carbo- 

 nate and horny matter, disposed in well-defined wavy 

 layers. PI. VI.' fig. 2. 



A. 35. Verrucella guadalupensis, with drawing of its structure. 

 The axis closely resembles that of A. 34, but its layers 

 are mostly parallel; occasionally spicules extruded from 

 the crust have modified the arrangement of the layers. 

 PI. VII. fig. 1. 



A. 36. Various portions of Isis hippuris, some in section, with 

 drawing of its structure. The axis is jointed, consisting at 

 first of short horny nodes alternating with long calcareous 

 internodes. As the axis increases in thickness the horny 

 matter is formed in excess of the calcareous, and may in 

 places overlap and conceal it. In the oldest parts of the 

 axis this sometimes is followed by an increased formation 

 of the calcareous material forming a continuous layer on 

 the surface. The ridges on the surface of the internodes 

 are continued inwards on the surface by which they are 

 attached to the nodes. The ectoderm-cells that secrete the 

 calcareous internodes are large and granular, with indistinct 

 nucleus. Those that secrete the horny nodes are about half 

 the size of the former, clear, and with a distinct nucleus. 

 PI. VII. fig. 2. 



G. v. Koch, Morph. Jahrb., Bd. iv. 1878, p. 116. 



A. 37. Plexaurella crassa, with drawing of the structure of the 

 axis and crust-spicules. The solid axis consists of horny 



