POIA'CHAETA. 



473 



The Polychaeta are found fossil from the Silurian onwards. 



Development. There are two main types of embryonic develop- 

 ment met with in this group. In one of them the ovum is small 

 and has little food-yolk, the gastrula arises by invagination, and the 

 blastopore remains open as the mouth (Eupomatus uncinatus, 

 Pomatoceros triqueter) ; in the other the ovum is larger and has 

 inure food-yolk, the gastrula arises by epibole, and the blastopore 

 closes (Psygmobranchus proten&us, Nereis cultrifera, etc.). In all 

 cases hatching takes place at an early stage, and there is a prolonged 

 period of larval life. The larva has the troch.osph.ere form, which 

 has alread}^ been described in the case of Polygwdius (p. 453), or 

 a form which can be readily reduced to that of the trochosphere. 



The trochosphere of the marine Chaetopods presents two distinct 

 types: (1) larvae in which there is an extremely large blastocoele, 

 i.e., space between the ectoderm and endoderm, derived from the 



FIG. 378. Atrochal larvae. A, of Lumhrico- 

 nereis (after Claparede and Metschnikotf); 

 B, of Sternaspis sciitfttu (after Vejdovsky). 

 cu cuticle ; d intestine ; ent endoderm. 



Fir, 379. Mesotrochal larva 

 of Chaetopterus pcrgamen- 

 taccus (after Wilson). 711 

 mouth. 



segmentation cavity of the embryo ; (2) larvae in which there is no 

 blastocoele, the ectoderm and endoderm being in contact except 

 where separated by the larval mesoderm. 



Larvae of the first type, in all cases in which their embryonic 

 development is known, proceed from embryos with an invaginate 

 gastrula, i.e., from embryos which develop on the type first 

 described. The best known instances of this larval type are 

 afforded by Eupomatus, Pomatoceros, and Polygordius (Fig. 454). 



In like manner larvae of the second type, in all cases in which 

 the embryonic development is known, proceed from embryos with 

 an epibolic gastrula. As instances of this larval type, the larvae 

 of Psygmobranchus, Nereis, etc., described by Salensky, may be 



