118 HANDBOOK OF IXVEKTEBRATE ZOOLOGY. 



posterior end of that water-tube which is on the right side 

 in the figure may be seen to be united to the integument 

 of the dorsal surface of the body. Careful examination 

 will show that the body cavity is now filled Avith small, 

 transparent, branched connective tissue corpuscles, which 

 run across in all directions from the wall of the digestive 



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tr.'ict to the inner surface of the body wall. 



e. At the end of the next forty-eight hours, the larva 

 which is shown in ventral view in Fig. 68, has changed 



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its form, and the proportions of parts in several partic- 

 ulars, but the general structure is about the same. 



The mouth (wi), is now situated on the middle of the an- 

 terior edge of the oral lobe (), instead of on its ventral 

 surface, and a ciliated ridge- , with a prominence (#"), at 

 each end, has been developed along its ventral edge. 



The two pairs of pre-oral arms (a), and post-oral arms, 

 (c, c), are lengthened, and the tips of the latter pair are 

 almost covered with reddish-brown pigment spots. The 

 most marked change of form is due to the fact that the 

 lateral angles (w), between the two pairs of arms, have 

 travelled backwards nearly to the posterior end of the 

 body. 



The rudimentary arm (n), in this angle is scarcely larger 

 than it was at the last stage, but the rudiments of a fourth 

 pair of arms, the dorsal, lateral arms (<?), have appeared 

 between the angles and the pre-oral arms (') 



Careful comparison of the larva; at this stage with the 

 figures of earlier stages will show great changes in the 

 form and position of the spicules. The mass (p), formed 

 by the fusion of the posterior ends of the lateral spicules (c?), 

 is undergoing resorbtion, and is now much smaller than it 

 has been. The bar (e), which during the early stages 

 ran across the ventral surface close to the edge of the cil- 



