514 INVEETEBEATA CHAP. 



complete, therefore, the hydrocoele has the form of a hoop, recalling 

 its form iu Asterias and in Ophiothrix after metamorphosis has 

 commenced. The opening of the amniotic invagination becomes 

 first narrowed and then completely closed, so that it is converted 

 into a sac with a thick floor which is incorporated with the Echinus 

 rudiment, and a thin roof which is the amnion. 



The subsequent course of the metamorphosis consists mainly in 

 the enlargement of the Echinus rudiment. The hydrocoele soon 

 sends out five blunt lobes which protrude as finger-shaped processes 

 into the amniotic cavity. These are the rudiments of the five radial 

 water-vascular canals, and of their primary terminal azygous tube 

 feet (az.t, Fig. 388). In the intervals between them there are formed 

 five radiating ridges of ectoderm which are termed the epineural 

 ridges. The crests of these form two diverging lamellae, which 

 fuse with those of adjacent ridges and so roof in the epineural 

 canals covering the bases of the tentacles. These epineural canals meet 

 in a central epineural space, roofed over by a membrane termed 

 the epineural veil, which forms an upper floor to the amniotic cavity. 



From the left posterior coeloin, about the twenty-fourth day, there 

 are given off five pocket-shaped evaginations situated one under the 

 origin of each epineural ridge, into the substance of which it projects. 

 These evaginations are the homologues of the perihaemal pockets of 

 the Asteroid and Ophiuroid larvae ; they all originate from the left 

 posterior coelom, while it will be remembered that in the two other 

 larvae mentioned, one originates from the left anterior and four from 

 the left posterior coelom. About the same time it can be observed 

 that the left posterior coelom has sent out right dorsal and right- 

 ventral horns. They, however, are of small extent in comparison with 

 the bulk of the vesicle ; they soon meet each other anteriorly and so a 

 ring is formed, surrounding, not the oesophagus, but the stone-canal. 



The Echinus rudiment as thus described forms at first a com- 

 paratively small, star-shaped organ on the side of the larva, but as 

 development proceeds it becomes ever larger till it occupies the 

 whole side of the larva. This consummation is arrived at about the 

 fortieth day. 



The adult central nervous system is formed, of course, from the 

 ectoderm immediately covering the hydrocoele, and forming the 

 lower floor of the amniotic cavity. It can be first detected by a 

 thickening of this ectoderm and a great increase in the nuclei ; this 

 occurs as early as the twenty-first day. Later, fine fibrils can be 

 detected at the base of the ectoderm and can be traced into some 

 of the nuclei. 



As the Echinus-rudiment increases iu size, the tube feet become 

 longer and longer and project farther into the amniotic space. 

 Pointed spines make their appearance, rising from the upper floor 

 of the amniotic cavity. There are four spines in each iuterradius 

 arranged in the form of a lozenge. Each of these spines is a hollow 

 conical outgrowth of the ectoderm of the upp^r floor of the amniotic 



