Class POGONOPHORA JOHANSSON, 1937 



The Pogonophora are marine animals of sedentary habit, mainly inhabit- 

 ing the abyss, where they dwell in distinctive protective tubes, consisting 

 chiefly of chitin. 



The bilaterally symmetrical body consists of three segments — a small 

 protosoma furnished with a cephalic lobe and tentacles, a larger mesosoma, 

 and a very long metasoma — and internally the coelom is subdivided to 

 correspond with the outward segmentation. The unpaired coelomic sac of 

 the protosoma, the protocoele, communicates with the exterior by means of 

 a pair of coelomoducts, which fulfill an excretory function, and coelomic 

 canals lead from it into the tentacles. The mesocoeles, a pair of coelomic sacs 

 lacking coelomoducts, are situated in the mesosoma, and the paired meta- 

 coeles in the metasoma. The metacoeles, unlike the mesocoeles, have 

 strongly developed coelomoducts, acting as gonoducts. 



The nervous system is primitive, lying wholly within the dermal epithe- 

 lium. The central part, the "brain", is situated in the protosoma, and from it 

 run a median dorsal nerve trunk and tentacular nerves. 



There is no alimentary canal, mouth or anus whatsoever, and digestion is 

 performed by the distinctive tentacular apparatus. 



The vascular system forms a closed circulation, consisting of dorsal and 

 ventral vessels which communicate anteriorly through the system of ten- 

 tacular vessels, and posteriorly by means of transverse vessels. That part of 

 the ventral vessel which lies at the base of the tentacles is differentiated into a 

 muscular heart, which in some Pogonophora is bordered by a pericardial sac. 



The sexes are separate, without sexual dimorphism, and the single pair of 

 gonads lies in the metasoma. The sperm are confined within membranous 

 spermatophores which are furnished with long, thin filaments. The eggs, 

 rich in yolk, develop within the protection of the maternal tube. Cleavage is 

 total, unequal and bilateral, and neither a blastopore nor a secondary mouth 

 is formed. The coelom is formed enterocoelically. There is no free-swimming 

 larva and no metamorphosis. 



The class Pogonophora has two orders, the Athecanephria and the 

 Thecanephria. 



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