116 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



opening, fringed with cilia, and it has moved from its 

 primative position nearly to the anterior edge of the ven- 

 tral surface of the oral lobe. In a dorsal view it is visible 

 through the oral lobe. 



The oesophagus (oe), is a long, thick-walled tube, lined 

 with cilia, and communicating, through a constricted open- 

 ing, with the large, globular, thick-walled stomach. The 

 intestine (), is an elongated, thick-walled tube on the ven- 

 tral surface of the stomach, but the pressure to which the 

 specimen shown in Fig. 65 w r as subjected has thrown it 

 onto one side. At this stage the mesoderm (we), may be 

 made out as a layer of irregular cells lining the body cav- 

 ity, and covering the digestive tract. 



On each side of the oesophagus, near the point where it 

 joins the stomach, notice a pair of small, transparent 

 bodies, the water-tubes (w). It is extremely difficult to 

 follow the history of these important parts in Arbacia, but 

 much easier to trace them in the transparent larvte of 

 Strongylocentrotus, where they may be seen at a much 

 earlier stage. 



d. A larva about twenty-four hours older is shown in 

 dorsal view in Fig. 67. The shape of the body is about 

 the same as in the preceding stage, but as Fig. 65 was 

 drawn from a compressed specimen, while Fig. 67 was 

 drawn from one which was swimming freely in the water, 

 the figures show considerable difference of outline. 



At this stage, notice especially the increased length of 

 the pre-oral (', '), and post-oral (c, c), arms ; the formation 

 of a little prominence (.), on each side in the angle between 

 these arms ; the increased length of the dorsal tran.-\ er>e 

 spicules (?), which were only small spines at the last stage ; 

 the division of the oesophagus into two chambers ; and the 

 lengthening of the water-tubes (w). At this stage, the 



