Relationship between Dictyocotyle and Calicotyle

Abstract

IN giving our opinion that ‘Dictyocotyle’ is simply a cœlomic form of Calicotyle we were well aware of some apparent structural differences between the two forms. Indeed, some of the supposedly distinctive characters had been detailed^1,2. Perhaps the most obvious difference is one of size; Calicotyle rarely exceeds 5 mm. in length, but ‘Dictyocotyle’ may exceed 11 mm. It is not surprising, therefore, that the eggs of the cœlomic form should also be larger. However, the difference is not great, even in the imperfect state of our knowledge. One of us gave the size of the egg of the cœlomic form as 186µ × 100µ, with a filament 19µ long²; Hunter and Kille³, who found many eggs but did not give the size-range, provided the figures 215µ × 100µ (+ 50µ for the filament). In the places cited by Llewellyn, for Calicotyle, comparative figures are 100µ × 110µ (+ 35µ for the filament)⁴ and 100–110µ (+ 100–130µ for the filament)⁵. As regards shape, which defies description and must be greatly influenced by fixation, the eggs of the cœlomic form were said to be “pyriform”², “roughly spindle shaped, and triangular in cross section”³, while those of Calicotyle were described as “triangular in optical section”⁴, and “tétraédriques” (“deux côtés du tétraèdre sont parcourus par une lame mince formant une carène”)⁵. The “three-sided pyramidal” eggs of C. affinis measure 145µ × 104µ (+ filament 52µ long) (no range given)⁶. Incidentally, the “carène” mentioned by Euzet is indicated in photographs of the eggs of ‘Dictyocotyle’³. Presumably the cœlomic form has a larger egg because of the greater size of the ootype, but we do not believe that significant differences in shape of the internal cavity of this organ, which acts as a mould in which the egg is formed, can be demonstrated for the cœlomic and cloacal forms in preserved whole mounts. As regards the cuticularized penis, the length is difficult to measure and may be less in forms acceptable to all as C. kroyeri than in ‘Dictyocotyle’, but it definitely is not less in ‘C. affinis’, where it likewise consists of three or more large turns of a spiral. Brinkmann⁶ is of the opinion that ‘C. affinis’ is a valid species (which is not proved beyond dispute) and a parasite of Chimaera which may wander from the cloaca into the rectum, but he found it also in the cloaca of Raia fullonica, in which location and host he also found C. kroyeri.