INTRODUCTION TO THE HIGHER INVERTEBRATES 239 



Mescntcrj/ 

 Coelom 



r^ores 



(Z.rve. co: 



rd 



r^Fla^dlafced 



protonephridiam 



Funnel 



Glandular 

 reOion. 



Mesoderm 

 band 



Schizocoeloin 

 Mesode-rm pouch 



Entcrocoe-Lom 



C D 



Figure 13.1. Coelom formation in the Eucoelomata. A, A diagrammatic cross section 

 showing the fully developed coelom and mesenteries. C, The schizocoelom. On the left 

 side is shown a solid band of mesoderm. A later stage is shown on the right (C), in which 

 a cavity has appeared. D, The enterocoelom. The mesoderm and pouch are shown form- 

 ing on the left side. \ later stage is shown on the right after complete separation from 

 the gut. B, N'ephridia found in the eucoelomates. Protonephridia with one flagellum are 

 common in larvae, while adults often have the metanephridium, which opens into the 

 coelom (lower part of B). 



protonephridium, described in the chapter on flatworms. In the eu- 

 coelomates a different kind of excretory organ is common, the meta- 

 nephridium. This is a tubide open at both ends (Fig. 13.1, B), the 

 outer end opening as a nephridiopore and the inner end opening into 

 the coelom. The inner opening is a ciliated funnel that sweeps coelomic 

 fluid into the tubule. Within the tubule useful components of this fluid 

 are reabsorbed by a glandular region of the tube wall while the waste is 

 left and eventually ejected. In some forms additional waste may be ex- 

 creted by the glandular region, and if the metanephridium is intimately 

 associated with the circulatory system the funnel may be absent. 



Although metanephridia are the common adult excretory organs of 

 eucoelomates, many larval eucoelomates have protonephridia, usually 

 with the tuft of cilia replaced by a single long flagellum (Fig. 13.1). 

 This supports the idea that the higher invertebrates arose from the 

 lower. 



102. Spiral Cleavage and Its Evolutionary Importance 



The eucoelomates are also related to the lower invertebrates through 

 an embryonic process called spiral cleavage. In the Platyhelminthes, Ne- 

 mertea, Mollusca, Annelida, and several of the minor phyla, a pattern 



