436 EMBRYOLOGY 



an interradial plate. Accoi-ding to the observation of 

 Agassiz, the mouth arises by a shortening of the long 

 oesophagus, and the anus persists. 



The condition of the dorsal pore and the stone canal, as described by 

 Ludwig for Asterina, is interesting. In this form, after the separation 

 of the enteroccele and hydrocoele, a canal is developed on the latter, 

 which, attached to the water-vascular rosette, opens into the enteroccele 

 quite near the place where the dorsal pore is connected with the entero- 

 ccele. This is the stone canal, which does not unite with the dorsal 

 pore until later. Thus there is a stage in which the stone canal does 

 not open directly from the water-vascular ring to the outside world, but, 

 on the contrary, leads into the body cavity. This, however, is in turn 

 connected with the outside world by means of the dorsal jjore. Ludwig 

 compares this condition to that which he described for the Crinoids (Nos. 

 30, 32). In them the water penetrates by means of the pores in the cup 

 (Kelchporen) into the body cavity, to be taken from there and con- 

 ducted into the water-vascular ring by the stone canals, of which there 

 are several hanging down from the water-vascular ring into the body 

 cavity. 



The earliest fundament of the blood vasndnr system arises, according 

 to Ludwig, at the place where the intestine grows out to form the oeso- 

 phagus. In the mesenchymatous layer lying between the walls of the 

 hydrocoele, the enterocoele, and the intestine, there is formed a fissure, 

 which exhibits a lining of very flat cells. This is the fundament of the 

 first blood-vascular ring. The structure ordinarily described as the 

 central plexus of the blood- vascular system also arises as a fissure next 

 to the stone canal (comp. General Considerations, p. 456). 



The nervous system of Asterina is first established in the form of a 

 circular epithelial thickening, which surrounds the region of the future 

 mouth-opening. Its development is certainly similar to that of the 

 central nervous system of the Holothurioidea, with which we are already 

 familiar. 



The metamorphosis of those starfish larvae which differ from the Bipin- 

 naria- and Brachiolaria-forms, as, for example, that of Asterina (jibbosa 

 (Fig. 201, p. 421), is likewise accompanied by the transmission of the 

 greater part of the larval organs to the starfish (Ludwig). Only the mouth 

 and anus have to be formed anew, and the larval organ suffers degenera- 

 tion, being gradually absorbed. Here also the starfish arises from an 

 ainbulacral and an antambulacral fundament, which at first are separate. 

 The development of Pteraster vnlitaris seems to resemble that of 

 Asterina (Koken et Danielssen). In this starfish, however, a kind of 

 brooding occurs ; a membrane stretches out over the spines on the back 

 of the animal, forming a brood-chamber. Into this the eggs pass, and 

 there develop into the larvas and young starfishes. 



