THE VISCERAL MASS IN ANTEDON EOSACEUS. 307 



these sections, and also in those of later date, I have no means of 

 telling how much of it, if any, has regenerated and how much was left 

 after evisceration. 



The ambulacral system has begun to regenerate, as is shown by the 

 presence of new ambulacral pores. On examining one of these more 

 closely, it is seen to be vesicular, lined by columnar epithelium and 

 opening to the outside. No internal opening is yet visible. 



(b) This specimen 1 is much more advanced than the preceding one, 

 in that mouth and alimentary canal are already present ; in other 

 respects it agrees with it, so that it will only be necessaiy here to 

 speak of the alimentary canal. In the centre of the visceral mass is a 

 depression ; the epidermis covering the depressed area is thickened 

 and in the centre is a small mouth leading obliquely downwards into 

 a feebly developed alimentary canal. The alimentary canal has 

 already the characteristic form found in normal specimens ; that is to 

 say it takes about one turn in an almost horizontal plane and ends in 

 an anus placed interradially. The most remarkable feature about it 

 is the rudimentary condition of its walls. In histological characters 

 the walls of the alimentary canal are almost indistinguishable from the 

 roof of the visceral basin, and to all appearance have been formed by 

 an invagination of this roof. The roof of the visceral basin is com- 

 posed of two layers ; externally a thin layer of minute, rounded, deeply 

 staining cells which are probably epidermic, and beneath this a thicker 

 layer of connective tissue which stains less deeply. In the floor of 

 the central depression already mentioned, the epidermic cells show a 

 slight tendency to become columnar. The walls of the alimentary 

 canal are composed of exactly the same two layers, a layer of more 

 deeply staining cells which corresponds to the epidermic layer of the 

 roof, but is now, of course, on the inside, and outside this a layer of 

 connective tissue. The cells of the inner layer show a tendency to 

 become columnar, and this layer is more distinct than the epidermic 

 layer of the roof. The lumen of the alimentary canal is very small 

 and its walls are very thin and but slightly folded. There is no trace 

 of an anal cone ; the anus itself is represented by a minute perforation 

 in the roof of the visceral basin. The walls of the last part of the 



1 The great amount of regeneration which has taken place in this specimen compared with 

 the other of the same date suggests the possibility that some mistake may have been made, 

 either by myself or at Naples, with regard to the time of regeneration. There seems, how^ 

 ever, to be no doubt that considerable variation in this respect does occur. 



