The anatomy and histology of the nervous system and excretory system of the maldanid polychaetes Clymenella torquata and Euclymene oestedi

Abstract

The nervous system of the maldanid polychaetes Clymenella torquata (Leidy) and Euclymene oerstedi (Claparede) (= Caesicirrus neglectus [Arwidsson, '11‐'12]) retains its primitive association with the epidermis. It shows only slight metamerism in the presence of larger collections of neurones opposite the parapodia and of larger nerves at the segmental boundaries. Multicellular giant fibers are present in the ventral nerve cord; giant neurones which show a characteristic pattern of distribution in each species are also present. The cerebral ganglia supply nerves to the prostomial wall, nuchal grooves and the wall of the buccal cavity, and a pair of large nerves from the circumpharyngeal connectives also appear to join the buccal system. The organs of special sense are the elongated prostomial nuchal grooves, and prostomial ocelli in Euclymene but not in Clymenella. Statocysts are absent. Four pairs of nephromixia are present. They lie in the aseptate anterior trunk, in chaetigers 5–9 of Clymenella, and 6–10 of Euclymene. The nephridiopores lie at the ventral ends of the neuropodia of chaetigers 6–9 and 7–10, respectively. Each nephromixium consists of coelomostome, tubule and contractile bladder. The wall of the tubule and bladder consists of both excretory and ciliated cells. Most of the cytoplasm of the latter forms a bounding layer at the outer surface. The cytoplasm of the excretory cells contains lipid material and appears to synthesize lipofuscin. The tips of the excretory cells swell, fill with granules, and break off in the form of vesicles which are periodically expelled in clouds from the nephridiopores. Glycogen is present, especially in the ciliated cells of the tubule and coelomostome. Granules of a lipoid nature accumulate in (or between) cells of the nephridia, epidermis, and some regions of the gut, and may be excretory. Lipid granules also appear to be synthesized by coelomocytes which eventually end up in masses in the ventrolateral coelomic cavities of the tail. The nephridia act as gonoducts, but show no seasonal variation in either size or histological structure.

Capitellid connections: contributions from neuromuscular development of the maldanid polychaete Axiothella rubrocincta (Annelida)

“…This variety in the neuronal composition of the ventral neurite bundle in developmental stages is also known from other polychaete larvae and may either indicate distant phylogenetic relationships or merely reflect the recently suggested general wide plasticity of polychaete neural patterning and nervous system anatomy [31]. Apart from that, the general arrangement of the central nervous system is consistent with previous investigations of the adult maldanid and arenicolid neuroanatomy [21,39-42]. For example, no indications of ganglionic segmentation have been found in Arenicola [41].…”

“…(B) 3D reconstruction based on the CLSM image stack of the individual shown in A. (C) Semi-schematic representation based on histological sections, modified after Pilgrim [21]. Note the adapted designation of neuronal structures, the lack of a stomatogastric nerve ring (str), the presence of three instead of one anterior proboscis neurite (apn), the additional prostomial nerves (prn), the nuchal nerve (nn), and the nerves to the buccal epithelium (bun).…”

“…For example, no indications of ganglionic segmentation have been found in Arenicola [41]. Moreover, the description of the adult ventral nerve cord in the maldanid species Clymenella torquata [21] can be directly correlated with the observed threefold pattern of the ventral neurite bundle with one median and two lateral main strands in the tubulin, serotonin, and FMRF-amide stainings. It is highly probable that the lateral tubulinergic strands can be assigned to a dorsally located fibrous neuropil which is separated into two parts by giant fibers that run along the ventral midline.…”

“…Most studies of the internal morphology of maldanid polychaetes have focused on members of the subfamily Euclymeninae [13], to which also the investigated species, Axiothella rubrocincta , belongs. In particular, the comprehensive investigations of Pilgrim [16-21] serve thereby as a basis for comparison of our data on neuro- and myogenesis. We discuss the present data in the context of a hypothesized close annelid-echiuran relationship and contribute to the discussion on ancestral bodyplan features of the Annelida.…”

“…In this respect, it has to be taken into account that A. rubrocincta represents a sibling species complex with considerable plasticity between populations concerning reproductive mode, size, and feeding, but with no obvious morphological differences [22]. Herein, we have adopted most of Pilgrim's [19-21] designations. However, we use different terms for some neuronal structures and the anterior-most muscles in the head region due to incongruency of the macro-anatomical data described by Pilgrim, which are based on light microscopy, and our confocal microscopy data.…”