The Hooks of Taenioid Cestodes from North American Felids

Abstract

The cestode genus Taenia is composed of a number of closely allied species of which many are poorly defined and many are consistently improperly identified. Since most of the descriptions have been based on contracted specimens or upon worms that have been flattened under pressure and thus are markedly distorted (an extreme example is the type specimen of T. balaniceps Hall, 1910), there are very few species which can be readily identified on the basis of size, shape of the strobilus, or proglottid morphology. In addition, the hooks borne on the scolex, have been measured under various unstated conditions, and on the whole, have been surprisingly inadequate for diagnosing species. The literature dealing with Taenia, indicates very great variability in the size and shape of the hooks of most species. Stevenson and Engberg (1904) analyzed the variability in Taenia pisiformis Bloch, 1780, and demonstrated that the hooks in that species varied in size and shape even on an individual scolex. Similar studies have not been undertaken with other species and it is rather interesting to note that T. pisiformis can be characterized as a species in which the hook size and shape does vary, although specimens from Califomia, Utah, Pennsylvania and Tennessee examined in this study had large hooks 0.25-0.27 mm long. Other species do not seem to show as great extremes as those reported for T. pisiformis although the shape of the hooks of T. taeniaeformis Batsch, 1786, does show a marked variation, and yet the literature indicates that size variation is the rule. As stated above, the various authors who have reported hook sizes have not used constant methods. Some have measured hooks attached to the scolex, and have measured them en face or from the side; other workers have isolated the hooks and oriented them in one plane. These methods do not yield the same measurements, and in the main account for compilations of data resulting in keys in which the hook measurements, show a wide range, and overlap consistently. The measurements, and the illustrations in this paper were made from hooks teased from dissociated scoleces and mounted in poly-vinyl-alcohol. The method of measurement is that of Stevenson and Enberg (1904) and is commonly used. It is very difficult to orientate hooks in one plane even when using high magnification (144 diameters) with a binocular dissecting microscope. Many hooks roll or move after the coverslip is applied and thus the preparations used in this study seldom showed more than twenty percent properly orientated. All measurements were made, using a camera lucida, on a scale projected from a stage micrometer. The two, heavy-bodied species of Taenia from cats can sometimes be distinguished on strobigar characteristics. Taenia omissa Liihe, 1910, apparently