56 PART I. GENERAL ACCOUNT 



the net is developed somewhat more strongly, and in some species (e.g. in 

 Siboglinum caulkryi, Lamellisabella zachsi and Spirobrachia grandis) more or 

 less complete nerve rings may be developed in these places (Fig. 36C, D). In 

 the last two of these species both the anterior and posterior nerve rings 

 contain ganglion cells as well. Jagersten (1956) also found the posterior nerve 

 ring in Siboglinum ekmani and correctly emphasized that its presence in 

 Pogonophora is an important feature which they share in common with 

 Enteropneusta. 



Each tentacle is innervated by one nerve (Figs. 36, 55, 57-59) running 

 along its outer side in the thickness of the epidermis (Johansson, 1939; 

 Ivanov, 1958b). There are no nerve cell bodies in the tentacles. 



It has not proved feasible to investigate the innervation of the internal 

 organs of pogonophores. It is possible that this is carried out by the nerve 

 fibres of the cutaneous nerve layer penetrating through the basement mem- 

 brane of the epidermis, just as is found in Phoronis (Silen, 1954). Nervous 

 elements have not been found in the coelomic cavities in Pogonophora 

 (Ivanov, 1958b). 



The central nervous system of Pogonophora, like that of Hemichordata 

 and Chordata, is thus fundamentally a longitudinal dorsal nerve trunk. 

 Pogonophora differ from Enteropneusta, however, in the absence of any 

 ventral trunk in the metasomal region. The latter is always well developed in 

 Enteropneusta and in the metasoma has a greater physiological significance 

 than the dorsal nerve trunk (Bullock, 1940). We may agree with Jagersten 

 (1956) that the nerve ring encircling the body at the border of the mesosoma 

 and metasoma corresponds to the prebranchial transverse commissure of 

 Enteropneusta, which connects the dorsal and ventral nerve trunks. Further- 

 more, the cerebral ring of Pogonophora (the brain together with its ventral 

 commissure ) seems to correspond to the anterior nerve ring developed at the 

 base of the proboscis in many Enteropneusta (Knight-Jones, 1952). Finally, 

 in the Pogonophora, as in the Enteropneusta, the nervous system retains its 

 epithelial site. As regards the histological features of the dermal nerve plexus, 

 which have not been studied in any detail in Pogonophora, we may for the 

 present note only the general outline of the similarity with Enteropneusta. 



Amongst the essential differences between the nervous systems of the two 

 groups under comparison we may note in Pogonophora the absence of a 

 ventral nerve trunk, the absence of any nerve tube and the development of a 

 brain in the protosomal region. As is well-known, the dorsal nerve trunk of 

 Enteropneusta forms, in the region of the collar, a nerve tube detached from 

 the integument (Spengel, 1877; Shimkevich, 1889; Dawydoff, 1948). In 



