Excretory System

The pycnogonid literature, consisting of about 1,000 articles in 14 languages over the past century, contains only minimal references to an excretory system or states that these animals lack one altogether. However, Meisenheimer (1902), Dogiel (1911), and Loman (1917) found a suggestive gland in the first larval appendage of Ammothea echinata and Nymphon strömii, to which they did not attribute any function other than a secretory one. Thompson (1909) also observed what he described as a nephrocyte in the same location in larval pycnogonids.

In the process of sectioning the anterior end of an adult specimen of Nymphopsis spinosissima (Ammotheidae), I found a conventional, though primitive excretory organ. It occupies the length of the scape of the chelifore and consists of an end sac, a straight proximal tubule, a short distal tubule, and a raised nephropore. The existence of this organ has been briefly mentioned in a prior publication (Fahrenbach, 1994).

The end sac, as seen in near-cross section of the scape (Fig. 1), is a thin-walled and polygonal chamber, about 150 µm in cross section, suspended in the hemocoel of the appendage, its edges radially guyed to the cuticle at half a dozen or more locations. This wall consists of an unadorned, in parts fibrous basement membrane, 1-4 µm thick, facing the hemocoel, and internally of a continuous carpet of podocytes and their pedicels (Figs. 2, 3). The podocytes, measuring maximally 10 x 15 µm, have a rich and varied cytoplasmic content: a few large mitochondria; large, dense and presumably proteinaceous inclusions that are obvious with the light microscope; a substantial Golgi system; and a labyrinthine system of connected intracellular channels that open into a central vacuole that often rivals the nucleus in size (Fig. 4).

Where they touch, podocytes are connected by adherent junctions. Substantial portions of the podocytes touch the basal lamina (Fig. 2), but they generally stand off from this surface and cover it with an uninterrupted layer of pedicels. These crowd together, often overlapping each other and forming a covering more than one pedicel deep. The gap between adjacent pedicels is crossed by an ultrafiltration slit membrane (Fig. 5).

The end sac fuses with the surface of the proximal tubule over a circle of about 100 µm in diameter (Figs. 6). In the center of this contact zone (Fig. 7), the two constituent epithelia lose their specializations and form a valve between them. The cells of the proximal tubule, a simple 150 µm long sac (Fig. 8), have not been characterized with the electron microscope, but they have a deep and irregular brush border and a thick epithelium of complex ap pearance.

The proximal tubule opens through a constriction, but no apparent valve, into the short (35 µm), cuticle lined distal tubule, actually a small chamber (Fig. 9). The opening of the nephropore has a raised cuticular rim projecting above the surrounding cuticle (Fig. 10). Musculature in the scape is largely restricted to the two ends and does not impinge on the excretory system. Only a few minor muscle cells are located within the region illustrated here.

Comments.

The end sac is bowed inward under the substantial blood pressure that the pycnogonids are capable of developing by virtue of their vigorous heart beat (90-180 beats/min; Tjønneland et al., 1985). Although the proximal tubule was not imaged with the electron microscope, its structure suggests close similarities with the same organ in, for example, lower crustaceans.

This excretory system has astounding similarity to that of primitive crustaceans, specifically to the maxillary gland of Hutchinsoniella macracantha (Hessler and Elofsson, 1991). Without delving into appendage homologies (Hedgpeth, 1954), it is tempting to compare the pycnogonid excretory system to the yet more primitive antennary gland of immature crustaceans, especially in view of its location in the first, preoral appendage. Conversely, the coxal glands of merostomates (Fahrenbach, 1999), scorpions (Farley, 1999), spiders (Felgenhauer, 1999), ticks (Coons and Alberti, 1999), and mites (Alberti and Coons, 1999) have substantial similarities with this gland, both with regard to position near the base of the first leg as well as ultrastructure. Thus, this gland, as seen in pycnogonids, can be considered representative of an ancestral arthropod excretory organ.

It remains to be seen where this gland is to be found in the numerous achelate species of pycnogonids.

References

Alberti, G., and L.B. Coons (1999) Acari: Mites. In: Microscopic Anatomy of Invertebrates. 8C: 515-1215. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Coons, L.B., and G. Alberti (1999) Acari: Ticks. In: Microscopic Anatomy of Invertebrates. 8B: 267-514. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Dogiel, V.A. (1911) Studien über die Entwicklungsgeschichte der Pantopoden. Nervensystem und Drüsen der Pantopodenlarven. Zeitschrift f. wissenschaftliche Zoologie, 98: 110-146.

Fahrenbach, W.H. (1994) Microscopic anatomy of Pycnogonida: I. Cuticle, epidermis, and muscle. J. Morphol. 222: 33-48.

Fahrenbach, W.H. (1999) Merostomata. In: Microscopic Anatomy of Invertebrates. 8A: 21-115. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Farley, R.D. (1999) Scorpiones. In: Microscopic Anatomy of Invertebrates. 8A: 117-222. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Felgenhauer, B.E. (1999) Araneae. In: Microscopic Anatomy of Invertebrates. 8A : 223-266. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Hedgpeth, J.W. (1954) On the phylogeny of the Pycnogonida. Acta Zool. 35: 1-21.

Hessler, R.R., and R. Elofsson (1991) The excretory system of Hutchinsoniella macracantha (Crustacea, Cephalocarida). J. Crust. Biol. 11: 356-367.

Loman, J.C.C. (1917) Beiträge zur Anatomie und Biologie der pantopoden. Tijdschrift der Nederlandsche Dierkundige Vereeniging 16: 53-102.

Meisenheimer, J. (1902) Beiträge zur Entwicklungsgeschichte der Pantopoden. I. Die Entwicklung von Ammothea echinata Hodge bis zur Ausbildung der Larvenform. Zeitschrift f. wissenschaftliche Zoologie, 72: 191-248.

Thompson, W. D'Arcy (1909) Pycnogonida. In: The Cambridge Natural History, 4: 501-542. S.F. Harmen and B.E. Shipley (eds.). London: Macmillan &Co.

Tjønneland, A., H. Kryvi, J.P. Ostnes, and S. Økland (1985) The heart ultrastructure in two species of pycnogonids and its phylogenetic significance. Zoologica Scripta 14: 215-219.

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