No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 



255 



The ectoderm cells in the vicinity of the blastopore form an 

 oesophagus which acquires a lumen, while the cells of the 

 entoderm are still in a solid mass (Figs. 63, 64). Considerably 

 later the proctodoeum is formed from cells within the paratrochal 

 ring which originally lay at the posterior lip of the blastopore, 

 and belonged to the somatic plate (p. 244). About this time 

 the mass of entoderm cells 

 acquires a lumen. The gut 

 is differentiated into a 

 stomach and intestine, and 

 in the walls of the former 

 is found nearly all the yolk. 



Fig. XVIII, p. 261, rep- 

 resenting a larva eleven 

 days old, shows that the 

 oesophagus, stomach, and 

 intestine are already highly 

 differentiated. The oeso- 

 phagus is ciliated through- 

 out, and there are numerous 

 strong cilia in the stomach 

 near the narrow cardiac 

 opening. 



Mesoderm. — When we 

 last referred to the mesoderm there were four cells 

 band (Fig. 61). For some time the number 

 simple division of the teloblasts, and then the single rows are 

 broken by cleavage of the component cells. Even after this, 

 however, one can distinguish the teloblasts lying close together 

 and as near as possible to the posterior end of the embryo (text 

 Fig. IX). In the much elongated larva one can make out in 

 section the mass of undifferentiated mesoderm at the posterior 

 end, and anteriorly the well-defined mesoderm layers lining the 

 gut and body-wall. There is no persistent primary body-cavity. 



Mucous glands. — After it is no longer possible to follow 

 exactly the arrangement of the other cells of the upper hemi- 

 sphere, the dorsal mucous glands gl.r. and gl.l. can easily be 

 traced, since their nuclei are particularly large and clear and 



Fig. IX. — Trochophore from the dorsal side, show- 

 ing prototrcch, paratroch, apical tuft, and meso- 

 derm bands within (stippled). From camera 

 sketch. 



in each 

 increases by 



