Structure and Development of the Nephridia. 347 



nerve cord, ave coutiinious with oue another and are well defined 

 iu tlie ectoderm of the anterior regiou of the body. Posterìorly, the 

 ventral cord is less distinetly delimited from the rest of the ecto- 

 derm, passing- gradually into the unditfereutiated tissue of the growìng 

 zone. Numerous lougitndiual muscle-fibres are closely applied to 

 the inner surface of the body-wall, and fib^-es derived from this 

 hiyer are already attached to the inner euds of the seta-sacs. No 

 circidar tibres, however, bave as yet appeared; these arise later 

 in development and apparently in an entirely dififerent manner 

 from the longitudinal fibrils, i. e. they appear in the ectoderm while 

 the longitudinal tibres are somatopleuric (see below). 



The free-swimming stage lasts for a period whose exact dura- 

 tion, in the case of an individuai larva, is difficult of determination, 

 but is probably from one to two days at normal summer temperature. 

 At its dose the larvae undergo the change which has been des- 

 cribed as a metamorphosis; they lose their cilia, sink to the 

 bottom, and adopt a crawling and burrowing and partially tubiculous 

 mode of life vv^hich lasts for the remainder of their existence. This 

 change takes place at about the time of formation of the fourth 

 setigerous somite. The larva represented in long:itudinal section in 

 figure 8 (Piate 22), is undergoing the transformation; it has already 

 lost most of its cilia; the mouth opening has broken through, and 

 the proctodaeum is in process of formation : the intestinal yolk is 

 evidently undergoing absorption, and the lumen has become more 

 distinct. 



The intestinal lumen quickly becomes spacious and continuous 

 from end to end of the body; and in larvae of 6 or 7 somites the 

 yolk has largely disappeared, and the, intestine has begun its func- 

 tional activity. The proboscis, by its continuai eversion and retraction, 

 fills the intestine with the surrounding débris, playing at the same 

 time an importaut part as a burrowing organ, in which form of 

 activity it is assisted by the action of the setae and the muscles of 

 the body-wall. From now on, the habits remain uniform and seem 

 in ali essential respects identical with those of the adult Arenicola. 

 The material introduced by the proboscis into the intestine is passed 

 through the latter, and the contained organic matter furnishes the 

 sole food-supply of the developing larvae. 



With the assumptiou of the definitive life-habits is associated a 

 chauge iu the reactions towards light and towards the contact of 

 solid bodies. The larvae become negative! y heliotropic, exhibiting 



