CHAPTER 6 

 The Coelom and its Derivatives — the Excretory System 



The extensive body cavity of Pogonophora, in which lie the internal organs 

 — the circulatory system, the coelomoducts, the genital organs and the 

 multicellular pyriform glands — is a true coelom. This is not only apparent 

 from the morphology of the adult animal but is confirmed by the embryo- 

 logical development (Ivanov, 1957b). The descriptions given earlier make it 

 sufficiently clear that pogonophores are segmented trimeric animals. The 

 coelomic cavities correspond with the segments. Each segment possesses its 

 proper secondary body cavity — an unpaired coelom in the protosoma and 

 paired coelomic sacs in the mesosoma and metasoma. Thus the Pogonophora 

 possess the same set of coelomic sacs as the Hemichordata and as the tornaria 

 and dipleurula larvae. 



The paired coeloms of the mesosoma and the metasoma come into contact 

 in the sagittal plane, forming a mesentery (Figs. 30, 45, 68). As in Hemichor- 

 data and Annelida the dermal musculature is formed from the sheet of tissue 

 external to the coelom — the somatopleura. All the coelomic cavities are lined 

 by a peritoneal epithelium, which also clothes the surface of the internal organs. 



The protocoele 



In comparison with the body cavities of the other two segments, the 

 invariably unpaired coelomic sac of the protosoma — the first coelom or 

 protocoele — is very small. It occupies a small part of the protosoma near the 

 base of the tentacular apparatus. The tentacular coelomic canals originate 

 from it and the afferent and efferent blood vessels of the tentacles run across 

 its cavity. The inner ends of the first pair of coelomoducts open into the first 

 coelom. In all the Athecanephria the coelom of the protosoma is a simple sac, 

 but in the Thecanephria it takes the shape of a horse-shoe whose ends curl 

 down onto the ventral side (Ivanov, 1955a, 1955c). The walls of the proto- 

 coele are formed of a very delicate peritoneal epithelium. 



The first coelom is especially small in Siboglinum where it is displaced 

 somewhat to the right side, presumably in connexion with the single tentacle 

 which is developed on the right side of the body in this genus (p. 25, 51); 

 even in the adult the tentacle remains attached just a little to the right of the 

 mid-ventral line (PI. I, facing p. 54), and the coelomic canal leading to the 

 tentacle pierces right through the right proximal dorso-ventral muscle bundle. 



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