6. THE COELOM AND ITS DERIVATIVES - THE EXCRETORY SYSTEM 63 



The protocoele is much more extensive at the end of embryological 

 development than in the adult animal (Ivanov, 1957b). Its front wall later 

 fragments on the formation of the musculature, however, and amongst its 

 derivatives are found the interesting dorso-ventral muscle bundles first 

 described by Caullery (1914) in Siboglimun rveberi and later bv Jiigersten 

 (1956) in S. ekmani. They have now been observed in all Pogonophora. 

 These muscles, which are most strongly developed in the Thecanephria, are 

 arranged at short irregular intervals in two longitudinal rows between the 

 median and lateral blood vessels (Pis. I, II). They join the ventral to the 

 dorsal body wall, with their ends inserted into the basement membrane of 

 the epidermis (Figs. 38, 52). Each bundle consists of rather thick smooth 

 muscle fibres with peripheral nuclei. To judge from the strong development 

 of these muscles they perform some important function. Their contraction 

 must flatten the cephalic lobe dorso-ventrally and so force blood out of the 

 extensive cephalic vessels. It may readily be supposed that their periodical 

 contraction may in large measure assist the flow of blood through these 

 vessels. This can scarcely be their only function, however, and it is possible 

 that by contracting rhythmically they push the blood from the cephalic 

 vessels into the dorsal vessel, thus effectively assisting the work of the heart 

 in propelling the blood along the ventral vessel (pp. 73, 74). 



The mesocoele 



The extensive coelomic sacs of the mesosoma — the mesocoeles — are 

 separated from the protocoele by a considerable space which is occupied 

 by a connective tissue and more or less widely spaced muscle fibres running 

 in different directions. The heart, part of the main blood vessels and the 

 whole or part (in the Thecanephria) of the protocoelomoducts also lie be- 

 tween the coelomic cavities of the protosoma and the mesosoma. No clear 

 dissepiments or muscular diaphragm can be discerned here (see Pis. I, II, III). 



The front ends of the mesocoeles reach the level of the heart and in the 

 Athecanephria are contiguous with the hind ends of the lateral cephalic 

 vessels (see Pis. I, II). The mesocoeles are separated from each other by a 

 delicate mesentery containing dorso-ventral muscle fibres and supporting 

 the dorsal and ventral blood vessels. In the Thecanephria the front ends of 

 the coeloms are secondarily pushed apart into a lateral position by the con- 

 siderable broadening of the dorsal blood vessels in this region and the in- 

 trusion of the coelomoducts of the protosoma between them (see PI. Ill, 

 facing p. 92). 



There are no mesosomal coelomoducts in any known pogonophore. 



