BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 61 



inside of the skeleton, ambulacra and interradia, and divides 

 with stalactite-like pillars, props, and partitions, the visceral 

 cavity into compartments. On the interradia! side of the an- 

 ricular branch this covering is smooth, adorally, at the base, 

 diverging sideways; on the ambulacral side it sends forth off- 

 shoots between the rows of pores, and aborally continues in 

 in a narrow ridge. The inner, adoral börder of each branch 

 presents a breach, a deep and narrow fnrrow running from 

 near the top towards the base and laying bare the interiör. 

 Its projecting margins give attachment to the motor muscles 

 of the pyramid, fl. VII, fig. 52—56. 



An auricle of Clypeaster reticulatus L., and 

 one of its branches showing the furrow. 



int. 



Transverse section of the auricle;* tlie core 

 int. the interradium. 



If a vertical section is made of a half-pyramid in the di- 

 rection indicated by the line B in fig. 58, the fossa a is laid 

 open with its auricle, and the retractor. This muscle, simple 

 in the regular Gnathostomes, is here bipartite, represented by 

 two separate parts, m. re. 1 and m. re. 2. In ftg. 62 this last 

 is to a great extent concealed beliind the auricle. When this 

 is removed, fig. 53, both parts are seen; when the auricle is 

 preserved, but the pyramid taken away, they are both visible: 

 fig. 54, from the interradial side: fig. 55, from above; and, 

 when a section is made through the pyramid by the line Ä 

 in fig. 52, and viewed in the direction of the arrow, as shown 



