PHYLUM ANNELIDA 



283 



130. The Relationships of Annelids, Molluscs and Arthropods 



Adult annelids and moUviscs differ markedly in appearance (com- 

 pare Figs. 14.2 and 15.1). Even if diagrammatic representations of the 

 phyla are compared (Fig. 15.9 A and C), they have little in common. 

 The annelid coelom is spacious (Fig. 15.9 B) whereas that of the molluscs 

 is small. The annelid "heart" is not a distinct organ; it includes the 

 dorsal blood vessel and often other vessels, whereas the molluscan heart 

 is compact. The annelid circulatory system is closed; that of the molluscs 

 includes extensive sinuses. Their nephridia, though basically similar, are 

 as different from each other as from the nephridia of many other 

 coelomate groups. The dorsal shell and ventral foot of the molluscs have 

 no counterpart in the elongate, annulated annelid. In short, a compara- 



Shell Uea^-rt ^' 



^0 -«?SS^.\o\^^\„.5^' 



— C oeiom. ~— 

 Parapodium. 



MefcanephridiuiTt- 

 Foot 





Figure 15.9. Diagrammatic representations of the Mollusca (A), Annelida (C), 

 Arthropoda (D), including a cross section of an annelid (B). 



and 



