Distylochus pusillus ( Bock, 1913 )

Bulnes, V. N., Faubel, A. & Park, J. - K., 2005, Two new marine species from South Korea with remarks on the family Stylochidae (Acotylea, Polycladida, Plathelminthes), Journal of Natural History (J. Nat. Hist.) 39 (23), pp. 2089-2107 : 2100-2101

Distylochus pusillus ( Bock, 1913)

( Figure 3F )

Stylochus pusillus Bock 1913, p 139 –141, Figures 22, 23.

Material examined

Voucher specimen. Nos 5965–67, three specimens sagittally sectioned, locality: Hong Kong, China, leg. K. Koch, 9 March 1862, deposited in SMNH .

Morphological notes

In the following, only morphological characters which deviate from the description of Bock (1913, p. 139–142) are added or re-described.

Reproductive system. Prostatic vesicle ( Figure 3F ) free, roundish oval, with finger-like extensions of interior glandular lining (polyglandular type). The interior lining is densely packed with glandular cells. Sparse extra-vesicular glands present in the surrounding parenchyma.

Discussion

In 1907, Meixner presented an account of the Stylochidae with an in-depth analysis of the structure of the different male copulatory organs. In his analysis, he defined two types of prostatic vesicle for the genus Stylochus : the neapolitanus type and the djiboutiensis type. The prostatic vesicle of the neapolitanus type is roundish to oval and contains few finger-like extensions which are long, directed distad rather than parallel, and originate from the proximal wall of the vesicle. The prostatic vesicle of the djiboutiensis type is elongate and contains numerous short tubes that are more or less radially arranged around the whole inner muscle wall. These definitions were accepted by Bock (1913), Palombi (1931), Steinböck (1937), Faubel (1983), and Newman et al. (1993). In following years, the lining of the prostatic vesicle was termed either chambered, ridged, or tubular, and adopted for the family Stylochidae .

In view of a new interpretation of the interior lining of the prostatic vesicle attained with the description of Cryptostylochus koreensis sp. nov. (see below), the grouping of the Stylochidae in species of djiboutiensis type and species of neapolitanous type is unsustainable. On the basis of that perception, the interior lining of the prostatic vesicle of Stylochus neapolitanus ( Delle Chiaje, 1841) , Stylochus alexandrinus Steinböck, 1937 , Stylochus frontalis Verrill, 1892 , Stylochus ellipticus ( Verrill 1873) , Imogine oculifera Girard, 1853 , Imogine nebulosa ( Girard, 1853) , and Distylochus pusillus Bock, 1913 were re-investigated. As a result of re-investigation of type material received from the Stockholm and Vienna museums, and reassessment of new and older descriptions, a completely new diagnostic interpretation of the glandular system of prostatic vesicles has been achieved.

The interior glandular lining of the prostatic vesicle in Stylochidae is composed of multiple finger-like extensions or tubes. Each tube contains a central glandular duct connected with a single extra-vesicular gland. Prostatic vesicles containing finger-like extensions, are supplied by several outlets of extra-vesicular glands along the epithelial glandular lining. These two types of prostatic vesicles within the Stylochidae , designated monoglandular type and polyglandular type, also differ with respect to the position of the glands relative to the prostatic vesicle. Monoglandular types generally have the glands in direct contact with the outer muscular wall of the prostate, where as the glands of polyglandular types are positioned at a distance from the prostate wall.

Within the Stylochidae , the genera Stylochus Ehrenberg, 1831 and Imogine Girard, 1853 remain separated on the basis of the presence or absence of a tripartite seminal vesicle, respectively. In most of the older descriptions of the Stylochidae , however, the structure of the inner lining of the prostatic vesicle was not, or inaccurately, described or only noted to belong to one of the types according to Meixner (1907). That means in most species described, the true glandular character of the prostatic vesicle is not to assess only on the study of the descriptions given in the literature.

Within the Stylochoidea , the families Stylochidae , Pseudostylochidae , Planoceridae , and Latocestidae are diagnosed by the possession of a fingered- or tubular-chambered interior lining of the prostatic vesicle. Among these families the Stylochidae appear most closely related to the Pseudostylochidae . The most conspicuous synapomorphies between these two taxa are the presence of a penis papilla, a ruffled pharynx varying in median parts of the body, and the absence of a cirrus. The autapomorphies of the Stylochidae and Pseudostylochidae lie in the presence or absence of extra-vesicular glands of the prostatic vesicle, respectively. This requires the emendation of the diagnosis of the Pseudostylochidae as follows: Stylochoidea with ruffled pharynx, more or less oblong in shape, and arranged in varying positions in the body but never at hindmost body end. Male copulatory apparatus directed backwards. Free oblong prostatic vesicle and with tubular chambered glandular lining, without extra-vesicular prostatic glands. Gonopores usually separate.

Figures

Figure 3. Diagrammatic reconstruction of prostatic vesicles of Stylochidae. Monoglandular types: (A–C) Cryptostylochus koreensis sp. nov.: (A) medio-sagittal section through the prostatic vesicle, (B) peripheral section through the prostatic vesicle, (C) diagrammatic reconstruction of the prostatic vesicle; (D) Cryptostylochus hullensis Faubel and Gollasch. Polyglandular types: (E) Stylochus neapolitanus (Delle Chiaje); (F) Distylochus pusillus Bock. Scale bars: 70 mm.