Dcvelojwioit vf llolothurians. 423 



On tlic ninth day the npive-rinf^- gives off Hvc tentacular 

 nerves, wliieh are interradial in orii^in and lie u]>on the mus- 

 cular hiycr of the tentacular vessels, on the side whicli is 

 towards the mouth. On the seventeenth day a nerve-branch 

 may be observed passing off from each side of the posterior 

 region of the median ventral radial nerve to the primary foot. 



As early as the eighth day of development the nervous 

 system of the young animal has no longer any connexion 

 Avhatever with the ectoderm of the surface of the body or of 

 tlie oral atrium ; it is everywhere separated from the ecto- 

 derm by an intervening layer of mesenchyma. Nevertheless 

 the outer surface of the nerve-ring and of the radial nerves 

 does not come into immediate contact with this mesenchyma, 

 but is separated therefrom by a cleft whicli persists tliroughout 

 the whole of the subsequent life as an " epineural ring " in 

 the case of the nerve-ring and as an " epineural canal " in the 

 case of the radial nerves. The epineural ring and e])ineural 

 canals are in free communication with one another from the 

 beginning; the latter are merely processes of the former. 

 On the other hand, a connexion between the epineural cavities 

 and any other cavity of the body could not be determined. 

 It ibllows from these observations that Ilerouard is perfectly 

 right in regarding the epineural ring and epineural canals of 

 the adult animal as normal structures. The tentacular and 

 pedal nerves are also accompanied by epineural spaces ; those 

 of the tentacular nerves branch off from the epineural ring, 

 those of the pedal nerves from the corresponding radial epi- 

 neural canal. 



Until the twentieth day the young radial nerves lie imme- 

 diately upon the outer walls of the radial vessels. It is not 

 until this day that — and at first, too, only in the median 

 ventral radius — a very fine cleft gradually appears between 

 the inner side of the radial nerve and the outer side of the 

 radial vessel. In all probability this cleft is the rudiment of 

 the subsequent radial " pseudo-ha?mal canal." As soon as 

 this cleft is formed, cells which are derived from the lateral 

 margins of the radial nerve pass to the outer wall of the cleft, 

 here to become the inner marginal cells of the perfect radial 

 nerve. 



On the other hand, I was unable to recognize, even in the 

 latest of the stages examined, either the perpendicular fibres, 

 or the transverse septum, or a trace of the two cellular columns 

 formed by the outer marginal cells, and therefore think I am 

 entitled to suppose that all these arrangements which are 

 known to exist in the radial nerves of the adult animal are to 

 be regarded as secondary acquisitions. 



