The cuticle of pycnogonids adheres to the general structural design
of the integument in other primitive arthropods. However, some
striking differences become immediately evident which have not been
previously observed. Since museum specimens provided good cuticle
samples that could be rehydrated, fixed for electron microscopy and
then processed, I solicited the cooperation of a number of
investigators in the field, to wit, Drs. J. W. Hedgpeth,
C. V. A. Child, R. Olson, and J. Staude, to provide me with preserved
samples of all families of pycnogonids. Cuticles from 20 species in 9
families were examined by scanning and transmission electron
microscopy. This material and methodology has been described
previously (Fahrenbach, 1994), an article that can be consulted for
Cuticle thickness ranges from 0.5 µm in the hindgut, 1-3 µm in the foregut, 30-50 µm in moderate-sized animals to a maximum of about 250 µm in Colossendeis robusta. It is composed of a thin epicuticle, several microns of exocuticle, a conspicuously layered and cross-banded endocuticle and the underlying complex-appearing epidermis.
In suitably and fortuitously stained specimens, the epicuticle consists of several thin strata, decorated with tubercles or ridges and sometimes covered with an evidently secreted layer that appears to protect the pycnogonid against epiphytic organisms (Figs. 1, 2).
The thin exocuticle, largely featureless in most preparations, displays typical swirling filaments in originally alcohol preserved specimens that obviously underwent some extraction (Fig. 2).
The substantive endocuticle is composed of several dozen laminae, each with the typical lamellar arrangement of filaments, which rotate in orientation progressively from layer to layer and display a conspicuous crossbanded pattern (Figs. 3, 4). The primary 21 nm periodicity can be resolved into a 12 nm light band and a 9 nm dark band (Fig. 5). This periodicity is displayed identically in the pure chitin pen of the squid Loligo vulgaris (Hunt and El Sherief, 1990), in all pycnogonids examined as well as untanned cuticles of many arthropods, especially lower crustaceans (Rieder, 1972a,b; Rieder et al., 1984). Where it has been found in higher crustaceans, chelicerates and insects, it is associated with thin, permeable cuticles found in respiratory or excretory surfaces. The universal lack of tanning among the pycnogonids allows them to increase substantially in size between molts. Flexibility of the cuticle at the site of arthrodial membranes is indicated by disappearance of the quasi-crystalline chitin structure, which imparts rigidity thereby.
The epidermis is basically a tall, columnar epithelium, but the cells are elaborately interfolded with each other and thereby suggest a many-layered structure (Fig. 14). Apically the cells abut the cuticle with stubby microvilli and a cleft of varying width, presumably indicative of the initial stages of ecdysis.
The cuticle is superficially decorated with spines and stout setae (Fig. 8), sensory setae with blunt termini, sensilla with one to six fine setae to each unit (Figs. 9,10), to complex, branched spiny projections (in Nymphopsis spinosissima).
Pycnogonids are unusual in that they have a diversity of cuticular glands. Copious pore canals are not readily visualized, but appear to receive their secretions from normal epidermal cells and deposit it on the surface as the epicuticle or one of its components (Fig. 6). Other glands are simple acinar glands well below the epidermis, connected to the surface by simple cuticular ducts, as in the salivary glands (to be described in comjunction with the proboscis). Other cuticular glands discharge through presumptive cuticle-derived shafts that each carry a number of ducts with complex cross-sections (Fig. 7). These ducts open preferentially through arthrodial membranes.
Finally, there is an enormous number of special mucus glands with a valvular opening, up to 1,400+ per mm2 (Fig. 11,12,) Depending on their size, they can be contained within the cuticle or spread out below it. The component cells resemble pancreatic acinar cells, i.e. they are filled with endoplasmic reticulum and large Golgi cisternae, indicative of a proteinaceous secretion rather than mucus composed of polysaccharides (Fig. 13). The cells open into a small lumen which communicates with the surface through a bivalved cuticular opening. The glands are commonly supplied with autonomic innervation, fibers containing large dense granules making occasional synaptoid contacts upon the gland cells (Fig. 13).
[N.B. A complete list of references to pycnogonid morphology, behavior and ecology, is contained separately in this website.]
Fahrenbach, W.H. (1994) Microscopic anatomy of Pycnogonida: I. Cuticle, epidermis, and muscle. J. Morphol. 222: 33-48.
Hunt, S., and A. El Sherief (1990) A periodic structure in the pen chitin of the squid, Loligo vulgaris. Tiss. Cell 22a: 191-197.
Rieder, N. (1972a) Ultrastruktur der Carapaxcuticula von Triops cancriformis Bosc. (Notostraca, Crustacea). Z. Naturforsch. 27b: 578-579.
Rieder, N. (1972b) Ultrastruktur und Polysaccharidanteile der Cuticula von Triops cancriformis Bosc. (Notostraca, Crustacea). Z. Morphol. Tiere 7: 361-380.
Rieder, N., P. Abaffy, A. Hauf, M. Lindel, and H. Weishäupl (1984) Funktionsmorphologische Untersuchungen an den Conchostraceen Lepestheria dahalacensis and Limnadia lenticularis (Crustacea, Phyllopoda, Conchostraca) Zool. Beitr. N. F. 28: 417-444.
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