CHJSTOPODA. 269 



closed ventrally the mesoblastic bands are still some little way apart on 

 the ventral side. 



In Euaxes the mesoblast originates in a manner which is very 

 similar to that in some of the Gasteropoda, e.g. Nassa, vide p. 193, 

 and Vermes, e.g. Bonellia, etc. As mentioned in the chapter on the 

 Mollusca the origin of the mesoblast in Plaiiorbis, p. 188, is very similar to 

 that in Lunibricus. 



Hatschek has shewn that in Polygordius the mesoblast arises in funda- 

 mentally the same way as in the Oligochseta. 



Besides the mesoblast which arises from the mesoblastic bands, there 

 is evidence of the existence of further mesoblast in the larve of many 

 Polychreta in the form of muscular fibres which traverse the space between 

 the body wall and the wall of the enteric cavity prior to the formation 

 of the permanent body cavity. These fibres have already been described 

 in the embryo of Serpula, and are probably represented by stellate cells 

 in the cephalic region (prse-oral lobe) of the Oligocheeta. These cells are 

 probably of the same nature as the amoeboid cells in the larva? of Echino- 

 dermata, some Mollusca and other types. 



TJie Larval form. 



True larval forms are not found in the Oligochseta where the 

 development is abbreviated. They occur however in the majority of 

 the marine Poly ch seta. 



They present a great variety of characters with variously arranged 

 ciliated bands. Most of these forms can be more or less satisfactorily 

 derived from a larval form, like that of Serpula (fig. 139 B) or Poly- 

 gordius (fig. 142); and the constant recurrence of this form amongst 

 the Chastopoda, combined with the fact that it presents many points 

 of resemblance to the larval forms of many Rotifers, Molluscs, 

 and Gephyreans, seems to point to its being a primitive ancestral 

 form for all these groups. 



The important characters of this larval form are (1) the division 

 of the body into a large pras-oral lobe and a relatively small post-oral 

 region containing the greater part of the alimentary tract; (2) the 

 presence of a curved alimentary canal divided into stomoda3um 

 (oesophagus), stomach and intestine, and opening by a ventrally 

 placed mouth, and an anus near the hind end of the body. To these 

 may be added the frequent presence of (1) a ganglion at the apex of 

 the pras-oral lobe, (2) a large cavity between the wall of the gut and 

 the skin, which is the remnant of the segmentation cavity, and is 

 usually traversed by muscular strands, of which one connecting the 

 apex of the prae-oral lobe and the stomach or oesophagus is very 

 commonly present (tig. 142). 



The arrangement of the ciliated bands presents great variations, 

 though in some instances it is constant through large groups. In 

 Chsetopods there is a widely distributed pra3-oral ciliated band, which 

 is similarly placed to the ring constantly found in the larvae of Mol- 

 luscs, Rotifers, etc. In many of these forms the band is practically 

 double, the opening of the mouth being placed between its two com- 



