CHAPTER 5 

 Nervous System and Sense Organs 



Nervous System 



It has already been stated that the nervous system of pogonophores is 

 wholly contained within the thickness of the cutaneous epithelium. In this 

 respect it compares with the nervous system of the Hemichordata and the 

 Phoronidea [and the Priapuloidea], and with the ectonervous system of 

 starfish and sea lilies. Histologically, the nervous system of Pogonophora 

 strongly recalls that of Enteropneusta (Ivanov, 1958b). 



The nerve layer is indistinctly divided from the rest of the epidermis, so 

 that it is rather difficult to delimit the region of the central nervous system. 

 To all appearances, there is often a continuous nerve net, with local thicken- 

 ings which are more or less rich in ganglionic cells (Figs. 23, 38). 



A voluminous nerve mass which may be termed the brain lies in the greatly 

 thickened epidermis of the dorsal side of the protosoma. Anteriorly it extends 

 far in front of the level of attachment of the tentacles, leaving clear only the 

 glandular epidermis of the anterior half of the cephalic lobe, while posteriorly 

 it reaches the hind end of the protosoma. At the level of attachment of the 

 tentacles the brain expands in lateral projections, reaching right round onto 

 the ventral side of the body where they meet, thus forming a complete nerve 

 ring. The nerves of the tentacles arise from the lateral parts of this ring. The 

 longitudinal unpaired dorsal nerve tract begins immediately behind the 

 brain and extends to the hind end of the body (Fig. 36). 



The antero-dorsal part of the brain is distinguished by its considerable 

 thickness and consists of two indistinctly divided layers — a basal fibrous 

 layer and an overlying layer of multipolar and unipolar nerve cells. The latter 

 lie scattered between the epidermal cells in small cavities (which may, 

 however, be artefacts), and are distinguished by their large nuclei and homo- 

 geneous cytoplasm which not infrequently contains Nissl bodies. There are 

 also spindle-shaped bipolar sensory cells with elongated oval nuclei. The 

 fibrous layer of the brain consists of thick longitudinal and transverse bundles 

 of nerve fibres. The basal parts of epidermal cells extend between the groups 

 of nerve cells and the bundles of fibres. There are no gland cells in the region 

 of the brain (Fig. 37). Amongst the fibre bundles the most powerful are the 

 dorsal transverse commissure, situated behind the cephalic lobe, and a pair 



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