Muscular System

Pycnogonid body musculature has been described by Dencker (1974) and that of the proboscis by Fry (1965); hence, this brief chapter will be restricted to the submicroscopic anatomy of the muscles. The fine structure of somatic and cardiac muscles has been treated by Tjønneland et al (1985), Totland and Kryvi (1986) and Fahrenbach (1999).

The fine structure of the muscles is that of striated arthropod design throughout the body, but three aspects stamp it as primitive compared to even that of microcrustaceans:

The basic design of the somatic myofiber has been detailed in the preceding paragraphs and is illustrated in Figure 1. Diameter of the cells ranges from 4-60 µm. Visceral musculature, like that of the pharynx, the digestive and reproductive systems adheres to the same ultrastructure. Muscles that cover the surface of the midgut are disposed as an irregular lattice of narrow strap-like fibers, about 1-3 µm wide and 0.5 µm thick.

The sarcoplasmic reticulum consists of standard dyads and triads facing the surface and the sarcolemmal infoldings, respectively (Fig. 2). Innervation consists of synaptic boutons depressed into the surface of somatic and visceral muscles without substantial synaptic modifications.

As a fiber approaches its insertion in the cuticle, it subdivides into numerous conical projections that interlock with corresponding recesses in the epithelial cells mediating the attachment (tendinal cell). The interlocking contacts between muscle and tendinal cell show septate desmosomal specializations. The tendinal cell is filled with robust microtubules that evidently provide the high tensile strength connection betwen muscle and cuticle. It should be pointed out here parenthetically that these “structural” microtubules differ from the standard “dynamic” ones, in that they fix well with osmium tetroxide and were seen long before tubulin microtubules were discovered after the advent of aldehyde fixation.

On the cuticular side of the myocuticular junction, the microtubules of the tendinal cells insert on membrane infoldings with opaque, thickened walls that each surround the origin of an anchoring filament. In some preparations, these can be followed into the endocuticle, where they provide the ultimate anchoring for the muscles akin to that of Sharpey’s fibers in vertebrate bone (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7).

Despite its vigorous action, the wall of the heart is extraordinarily thin, measuring generally no more than 0.5 µm in a small pycnogonid like Ammothea hilgendorfi and attenuating in places to about 50 nm. It consists of basal laminae facing both the cardiac lumen as well as the hemocoel and highly attenuated myofibers with minimal overlap and frequent stretches devoid myofilaments. The myofibers have internal and external projections, possibly as a result of contraction after fixation and loss of distending blood pressure and are about 2.5 µm long. The need for a sarcoplasmic reticulum is obviated by the reduced diffusion distances in this structure. Adjacent cells are connected by gap junctions (Fig. 8).

References
[N.B. A complete list of references to pycnogonid morphology, behavior and ecology is contained separately in this website.]

Dencker, D. von (1974) Das Skelettmuskelsystem von Nymphon rubrum Hodge, 1862 (Pycnogonida: Nymphonidae). Zool. Jahrb. 93: 272-287.

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

Fry, W. G. (1965) The feeding mechanisms and the preferred foods of three species of Pycnogonida. Bull. Brit. Mus. Nat. Hist. (Zool.) 12: 195-223.

Harrison, F. W. et al. (eds.)(1992-1999) Microscopic Anatomy of Invertebrates, Vols. 8 -11. Wiley-Liss

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

Totland, G. K., and H. Kryvi (1986) The fine structure of the somatic muscles and their attachment to the cuticle in two species of Pycnogonida. Zoologica Scripta 15: 69-72.

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