MONOGRAPH OF THE EXISTING URINOIDS. 



477 



On its first appearance in tiie embryo it is dorsal in relation to the hydrococle ; 

 in the free-swiniming larva it has become displaced in such a way that its chief 

 piece lies on the ventral side anterior to the hydroccele; in the stalked larva it 

 moves posteriorly in the same degree as the vestibule and hydroccele and comes 

 to lie horizontally always, however, maintaining its primitive relations with the 

 adjacent organs. 



The long anterior vertically running branch is very early withdrawn; it 

 becomes shorter and shorter, loses its tubular shape, and appears as a pointed 

 appendage of the transverse chief piece. This latter proceeds outwardly against 

 the originally ventral side after the vestibule end has been entirely withdrawn. 



While in the beginning its anterior end stretched to the end of the gut, and 

 in the free-swimming larva lay before it, nothing further is to be seen of it in 

 this region. 



Posteriorly as well as anteriorly it is considerably compressed dorsoventrally 

 and appears no longer as a transveree tube but as a sacklike structure which is 

 somewhat longer in the major axis than in the transverse axis. 



It appears to have increased in extent relatively much less than the other 

 organs. 



Since the chief part of the parietal canal has moved further posteriorly it 

 has approached the plane of the pore, although always somewhat anterior to it. 

 The ascending branch becomes more and more, though never quite, horizontal. 

 The end piece runs approximately crosswise. While the outer end bends abruptly 

 and opens outwardly through the pore, the inner end passes insensibly into the 

 parietal cavity. The pore lies near a sacculus, a little ventral to radius I. 



Histological!}' only the portion of the end piece adjoining the pore differs 

 from the conditions found in the parietal cavity, which is lined throughout with 

 a pavement epithelium at this stage everywhere demonstrable and nowhere show- 

 ing a fusion with the mesenchyme cells. Further toward the oral side, about the 

 stone canal, the cells are mostly a little thicker and the epithelial character of 

 the wall is more easily apparent. The extreme end, which makes an obtuse angle 

 with the remaining portion and breaks through the ectoderm layer of the body 

 wall, consists of rather higher cells. Although the lumen is more extensive than 

 in the adjoining transverse canal, its diameter as a rule is less than that of the 

 wall. In cross sections the end piece and its lumen are circular. On the inner 

 side the cells bear a cuticlelike lining finely striped transversely and bearing cilia. 



MESENCIIYJtE. 



Calcareous skeleton. — As long as the vestibular invagination remains on 

 the ventral side in the embryo, and in the free-swimming lai-va, the five basals 

 and five orals lie in two horseshoes, of which the two arms are rather widely 

 separated ventrally. When the vestibule has finally assumed its appointed place 

 on the posterior end of the body the two .series have rearranged themselves in two 

 pentagonal systems at right angles to the major axis, directly superposed. 



