The Development of Phascolosoma. 221 



presents a bulbous enlargement in front (Fig. 72, 73), which is due 

 to the contraction of the circular muscles of the walls of the middle 

 and posterior portions of the body. This movement is characteristic 

 of the adult in its locomotion by burrowing-, and serves to fix the 

 anterior end of the body in the burrow, previous to drawing up 

 the posterior extremity. 



Although there is now little locomotion when the animal rests 

 on a glass surface, the introvert, which includes the anterior third 

 of the body, is repeatedly drawn into the body cavity by successive 

 retractions of this region. This is brought about by the contraction 

 of the retractor muscles, and is accompanied by a contraction of 

 the longitudinal muscles of the body wall, a relaxation of the circular 

 muscles of the post-anal region, and a backward flow of fluid in the 

 body cavity. During an introversion the supraoesophageal ganglion 

 comes to lie opposite, or even behind, the anus (Fig. 78). 



Evagination of the introvert is produced by the contraction of 

 the circular muscles of the postanal part of the body, whereupon 

 the fluids of the coelom are forced forward and, since the retractoi^ 

 and longitudinal muscles of the body wall relax, the anterior part 

 of the body is forced forward. 



Third week and later (Fig. 56, 57, 72—78). 



The yolk is almost completely absorbed by the end of two 

 weeks, and from that time onward the larvae, which, were kept in 

 an aquarium containing merely pure sea- water, and were thus deprived 

 of their natural nutriment which is contained among the particles 

 of muddy sand, did not increase much in size. From 0,5 mm to 

 0,6 mm in length at fourteen days, the larvae grew to the size of 

 0,75 mm at twenty days, after which they did not increase materially 

 in length. 



The larva from seventeen or eighteen days onward has a con- 

 striction of the oesophagus, at about one quarter of the length of 

 the body behind the mouth (Fig. 56, 57, 72, 73). A pair of delicate 

 muscles, consisting each of only one or two cells, which extend from 

 this region to the ventro-lateral part of the body wall, now make 

 their appearance (Fig. 73, 77, 78 nm. oe). A similar muscle attaches 

 the rectum to the body wall (Fig. 78 m«. ret). Both the pharjmx 

 and the anal part of the intestine are ciliated (Fig. 71, 73). The 

 anterior and posterior ends of the intestine have a markedly smaller 

 diameter than the middle part, which is coiled up in the posterior 



