146 PHYLUM ECHINODERMATA. 



so that these mesenteries are really remains of the septum between the 

 right and left posterior coeloms. In the central region of the disc this 

 septum seems to have disappeared in the adult. 



The anterior coslom, with the atrophy of the preoral lobe, becomes much 

 reduced in size. As stated above, it persists as the axial sinus. The 

 inner perihaemal ring is an outgrowth of the axial sinus, with the ventral 

 end of which it remains continuous (Fig. 105, int.p.r.). 



The axial organ makes its appearance as a ridge projecting 

 into the axial sinus and containing jelly, fibres and leucocytes. 

 Later there is formed a thickening of the epithelium of the left 

 posterior coelom near the aboral end of the incipient axial organ. 

 The cells of this thickening are the primitive germ-cells. They 

 become invaginated into the septum separating the axial sinus 

 from the left posterior coelom, and thence grow out in two 

 directions, viz. (1) into the ridge of the axial organ forming its 

 core, and (2) as a cord extending in a direction parallel to the 

 surface of the disc and constituting the generative rachis. The 

 generative rachis becomes enclosed by a flap of peritoneum in 

 a space cut off from the left posterior coelom and known as the 

 aboral sinus. It forms an aboral ring on the dorsal side of the 

 stomach and sends off, as it passes each interradius, two branches 

 enclosed in corresponding branches of the aboral sinus. These 

 branches end in swellings which become hollowed out, acquire 

 a communication with the exterior and form the generative 

 glands. The portion of the aboral sinus round the gonads be- 

 comes cut off by a septum from the rest. 



The metamorphosis of Asterina gibbosa begins on the eighth 

 day of its development. The larva fixes itself by its preoral 

 lobe to the substratum by means of a thin mucilage which 

 appears to be secreted by the adhesive disc, and it remains 

 attached during the whole of the metamorphosis (Figs. 102 to 

 104). The following changes occur : (1) The constriction of 

 the body into disc and stalk, the latter being formed from the 

 preoral lobe. (2) The sharp flexure of the disc on the stalk, the 

 disc being bent obliquely and to the left, so that the left side of 

 the body is turned towards the substratum. (3) The prepon- 

 derating growth of the organs of the left side, the left posterior 

 coelom and the left hydrocoel having both sent out dorsal and 

 ventral horns, which meet so as to form complete circles, while 

 the right hydrocoel and the right posterior coelom remain 

 small. (4) The gradual atrophy of the stalk. (5) The out- 



