PHYLUM ANNELIDA 



285 



indicates that the similarities of adult structure are associated with 

 similarities in development. In the annelids (Fig. 15.10 D) the meso- 

 derm, which remains as a pair of bands in the trochophore, elongates 

 and becomes divided into pairs of somites. Within each somite a 

 coelomic cavity appears. The somites of each pair expand dorsally and 

 ventrally around the gut, eventually forming a ring with dorsal and 

 ventral mesenteries. To complete the process of segmentation the body 

 wall constricts between adjacent rings. The body elongates during this 

 process, and segmentation begins at the anterior end. In arthropods 

 the mesoderm follows a similar pattern of development, starting as a 

 pair of longitudinal bands that become divided into somites, with 

 coelomic cavities appearing in each somite. Later the cavities disap- 

 pear, but the somites correspond with the segments of the adult body. 

 In both phyla the ventral nerve cord arises from the midventral line as 



Esophagus 



Mouth 



Protonephridiuin 

 Mesoderm band.' 



Apical organ 

 Brain rudiment 



Stomach 

 ^[_f Profcot roch 



Anas 



le/— Anus 



Figure 15.10. Development in annelids and molluscs. A, A typical trochophore. B, 

 Diagrammatic representations of the development of a mollusc from a trochophore. C, 

 The same for an annelid. D, Ventral views of the gut and mesoderm bands of an annelid 

 from the trochophore stage (left) through the formation of a few anterior segments. 



