10. EMBRYOLOGICAL DEVELOPMENT 



117 



Enteropneusta, both at the tornaria stage and during the early stages of meta- 

 morphosis, the region of the metasoma is very tiny and for a time remains 

 much smaller than the proboscis and collar, but then it rapidly increases in 

 length. The grooves between the segments usually arise simultaneously, but 

 in Saccoglossus pusillus (which is very like Siboglinum in the amount of yolk 

 in the egg, in the way coelomic cavities are formed and in the lack of a pela- 

 gic larval stage) the metasoma is divided off first (Davis, 1908). 



The ciliated girdles appear to be provisional formations. In adult pogono- 

 phores there is no trace of them, though it is not clear whether there is any 

 developmental connexion between the posterior ciliated girdle and the dorsal 

 ciliated band on the trunk of the adult. Two hypotheses may be advanced to 

 account for the possible functional significance of the ciliated structures : the 

 cilia might facilitate exit from the maternal tube or assist in respiration by 

 drawing a current of water over the developing embryo. The first hypothesis 

 is less likely since the whole ciliary system, with its very short cilia, seems 

 hardly strong enough to propel the animal to the outside. [The late embryo 

 of Siboglinum inerme, removed from the maternal tube, has been seen to 

 swim by means of the ciliated girdles (see PI. IV D) — D.B.C.] 



The two girdles, together with the dorsal ciliated area of the mesosoma, 

 apparently represent remnants of the larval locomotor system preserved 

 from the time when ancestors of the Pogonophora possessed a free-swimming 

 larval stage. In particular, the metasomal girdle is probably homologous with 

 the posterior ciliated ring of the tornaria, which 

 first appears in this larva after all the other cilia 

 have been formed. 



Amongst the provisional "larval" organs one 

 must also include the bristle sacs and their bristles. 

 To all appearances the bristles serve to support the 

 body against the internal wall of the tube when the 

 young animal is creeping forth and moving along by 

 means of contractions of the dermal musculature. 



A very interesting point is the way in which the ten- 

 tacles arise one after the other, not simultaneously, 



ps—W 



ms 



Fig. 83. Late embryo of Siboglinum sp., seen from the dorsal side. 

 ca - anterior ciliary girdle ; с h - provisional bristles of metasoma ; 

 cid - ciliated field of mesosoma ; cp - posterior ciliary girdle ; ms - 

 mesosoma ; mts - metasoma ; ps - protosomal region ; x - pro- 

 visional bristles of mesosoma; у - terminal invagination of 

 integument. (After Jagersten, 1957; orientation of body and 

 interpretation of segmentation changed.) 



mts- 



