14 PHYSIOLOGICAL SERIES. 



those of calcareous sponges, are optically as if carved from 

 a crystal of calcite, but their forms are organic and often 

 closely resemble siliceous spicules. 



Stewart, Trans. Linn. Soc., vol. xxv. 1866, p. 365. 



A. 46. Two halves of a Cake-urchin (Clypeaster humilis). Pro- 

 cesses of the test arise from the under (actinal) and upper 

 (abactinal) body- walls, the former being twice the length 

 of the latter ; they meet and firmly articulate without 

 fusion. They must greatly increase the strength of the 

 test. There are also fine needle-like processes where the 

 intestine is lodged, and five pairs of processes around 

 the mouth (peristomal interambulacral apophyses). 



Presented by Prof. C. Stewart. 



A. 46 a. Test of a Cake-urchin (^Clypeaster rotundus), the upper 

 (abactinal) portion being removed, and the jaws (pyramids) 

 left in situ, The strengthening wall-like columns are con- 

 fined to the margin of the test. They are grouped into five 

 interradial sets. In each set the walls bound a central 

 area that opens towards the vertical axis of the body, and 

 has processes extending towards its interior. From the 

 outer sides of the wall bounding the central area extend 

 more strongly developed walls mostly parallel to the margin 

 of the test ; these are separated from similar walls of 

 neighbouring sets by about 3 mm. In this form the con- 

 necting pillars and processes, the five processes (peristomal 

 interambulacral apophyses) around the mouth, and the five 

 jaws (pyramids) constitute the endoskeleton. 



A. 47. A Sea-cucumber (Holoihuria nigra), laid open. The 

 mouth is surrounded by a ring of ten calcareous plates, five 

 radial to which the longitudinal muscles of the body are 

 ' attached, and five interradial. 



Presented by Prof. C. Stewart. 



ANNELIDA. 



A. 48. A tube-dwelling worm (Spirographu spallanzami), show- 

 ing the branchial skeleton of the right side. The main 

 portion of the skeleton consists of two tapering ribbons of 



