370 Mr. H. Seeley on Cambridge Paleontology : — 



the ventral covering being gone. The depth which this visceral 

 groove extends into the lobe varies greatly with the species. It 

 is always wide near the attachment, and gradually narrows and 

 becomes less deep ; but it does not terminate in a point, there 

 being a little ledge at the end, forming a notch, in which it is 

 extremely probable that the little ganglion for the eye was placed. 

 It is to be noticed that towards the end the groove curves up- 

 wards towards the dorsal surface, as do the ambulacra of many 

 star-fishes. The object of this is evident ; for, from the solidity 

 of the rays, they were incapable of motion : so, had the feet 

 been confined to the ventral surface, the animal could only have 

 moved on flat ground; but by the beautiful arrangement of 

 their reaching in a curve some distance up the anterior side, the 

 want of motion is in a measure compensated for, since by it 

 they could ascend any elevation that might be met with. The 

 ambulacra appear to have differed from those of the star-fish in 

 the remarkable character that the feet were not protruded through 

 bony plates ; for there was no trace of them in any of the speci- 

 mens examined ; and on microscopically examining the contents 

 of the visceral groove in some very perfect specimens, the only 

 structures visible were a few minute and thin square and rhom- 

 boidal bodies. It is possible that these may have been imbedded 

 in the membrane through which the feet were protruded. 



In many specimens the ambulacral groove is seen to be marked- 

 transversely throughout its length with little striae, which, com- 

 mencing in a puncture just inside of the margin, extend across 

 from side to side. They are very much coarser and wider apart 

 than the fine strise observable on the jaws of Echinoidea ; but it 

 would appear very probable that, as in those structures, they 

 indicate the attachment of muscles. Judging from analogy, it 

 may be conjectured that this great muscle was a retractor, draw- 

 ing the animal's body closer to that surface to which the feet 

 were attached. 



Margining the grooves on each side is a row of large tuber- 

 cles, which are generally surrounded by pits, and have a fissure 

 on the summit for the insertion of the ligament of the attached 

 spine. Spines, generally minute, margin the ambulacra of Star- 

 fishes, and gradually become large among the Sea-urchins. 

 There can be no doubt that they were formed to protect the 

 suctorial feet ; and as it has already been seen that these were 

 protruded through a membrane, the necessity for a more efficient 

 protection than that usually given to star-fishes will be evident. 

 If the large size of the articulation may be taken as an indi- 

 cation of the length of the spine, it may be presumed that the 

 suctorial feet were capable of being protruded to a great 

 length. 



