426 On the Develo'pmint of Tlolothurians. 



into a distinct epitlielium and a subjacent layer of connective 

 tissne. 



Blood-vascular System. — The supposition tliat the blood- 

 vascuhnv system, as I was the tirst to demonstrate in the case 

 of a starfish, would be traceable to remnants of the seg-men- 

 tation-cavity, or at any rate to clefts in the mesenchyma, has 

 fully justified itself. Between the visceral layer of the ente- 

 rocoele and the endoderraic wall of the mid-gut there appears 

 on the thirteenth day a distinct space, which partly bulges 

 out to form the marginal vessels of the perfect intestine and 

 partly develo|)s into the blood-spaces which are found in the 

 thickness of the wall of the mature intestine. On the seven- 

 teenth and eighteenth days we can already observe the 

 development of a mesenterial and an antimeseuterial marginal 

 vessel upon the mid-gut, to which during the following days 

 a simpjle transverse vessel is added. 



Just as between the visceral layer of the enteroccele and 

 the endoderm of the mid-gut, so also, in a similar way, lacunar 

 vessels are developed between the parietal layer of the ente- 

 rocoele and the mesenchyma of the body-wall. [Since a firm 

 and intimate fusion of the parietal enteroccele with the body- 

 wall takes place in the region of the radii only, in the inter- 

 mediate spaces, that is in the interradii, a gap remains between 

 the enteroccele and the body-wall, which may be detected 

 even in quite young stages, and is identical with the large 

 lacuna of tlie body-wall described by Herouard in the adult 

 animal. 



Digestive Organs. — The oral atrium already alluded to is 

 clothed by a very flat unilamellar epithelium, which is directly 

 continuous with the external covering of the tentacles. At 

 the bottom of the oral atrium lies the opening of the mouth, 

 which on the eighth and ninth day is extraordinarily narrow 

 and takes in no food as yet. The folding of the intestine, 

 subsequently so strongly marked, is already indicated on the 

 ninth day, and from the beginning follows the same regular 

 direction as in the adult animal. The fore-gut narrows pos- 

 teriorly, and on the twelfth day is already attached by means 

 of fine radial strands of connective tissue to the inner side of 

 the young calcareous ring. Not less distinct and much more 

 numerous are at the same period the suspensory cords which 

 attach the hind-gut to the body-wall. On the fifteenth day 

 the mid-gut has widened considerably ; tlie fore-gut is now 

 marked off from it by a sharp constriction. On the seven- 

 teenth day I was able to observe food (Diatoms) in the mid- 

 gut, derived from without, although at this time tlie food- 

 sup])ly stored up in the gelatinous nucleus of tlie ceplialic 



