Lagenopolycystis Artois & Schockaert, 2000

General description of the taxon Lagenopolycystis Artois and Schockaert, 2000 (= Lagenorhynchus Brunet, 1965 )

( Fig. 1 View FIGURE 1 )

Species of Lagenopolycystis are relatively small, measuring approximately 0.8–1.5 mm in size. Specimens are mostly transparent, with the anterior end often slightly orange-tinted.All species have relatively big eyes. The pharynx is of the normal polycystidid construction ( Meixner, 1925) and is positioned a little anterior to the body middle ( Fig. 1A–C View FIGURE 1 ).

The proboscis was described in detail by Brunet (1965), who mentioned some anatomical peculiarities compared to other polycystidids. However, our observations show that its anatomy is almost identical to that of species of Typhlopolycystis (see Brunet 1965; Schockaert et al. 2019) and, by extension, that of other representatives of Typhlopolycystidinae , e.g. Myobulla Artois & Schockaert, 2000 (for a detailed description and illustration, see Artois & Schockaert 2000). It is about 1/4 –1/5 of the body length, with the cone retractors parallel to each other and a relatively thick inner circular muscle layer. The proboscis is a slender and elongate organ, with a pointed foreend and a very small apex. As in all typhlopolycystidinids, there are four pairs of proboscis retractors, one pair of integument retractors, and two pairs of strong dilatators on the proboscis cavity.

The gonads are unpaired and (mostly) positioned at the left side of the body. Vitellaria can be paired or unpaired. The genital pore is at about 80%; the genital atrium receives the uterus anteriorly, the male atrium from above and the female duct from behind.

As in all members of Typhlopolycystidinae , the female atrial organs ( Figs 1C–F View FIGURE 1 ) consist of a short, moderately muscular female duct (type I) that receives the resorbing terminal bursa from behind and the ovo-vitelloduct anteriorly. Next to the entrance of the ovo-vitelloduct, there is a small sclerotised tube (tsr in Fig. 1 View FIGURE 1 ), with a strong sphincter at its base. Such a sclerotised tube is also present in species of Limipolycystis , which exhibit an identical construction of the female atrial organs, apart from the fact that in species of Limipolycystis , this tube ends in the proximal part of the oviduct, while in species of Lagenopolycystis , it ends freely in the parenchyma (see Artois et al., 2012). This tube is usually very difficult or impossible to observe in live as well as in whole-mounted specimens. Often, sperm can be seen converging in the direction of this tube ( Figs 1E–F View FIGURE 1 ). Brunet (1965) initially interpreted the robust sphincter at the base of the tube as a closure for the bursa, inadvertently overlooking the presence of the tube itself. The function of the tube remains enigmatic, and it may serve as an insemination duct, or it could be involved in the evacuation of excess sperm to the parenchyma or intestine. In species of Typhlopolycystis , Brunetorhynchus , and Sabulirhynchus , a pear-shaped muscular ‘seminal receptacle’ occurs instead of this sclerotised tube. Given the fact that Artois and Schockaert (2005) considered the tube to be homologous with the seminal receptacle, we will continue to refer to the sclerotized tube as the “sclerotized tube of the seminal receptacle”, expressing this hypothesis of homology.

The male atrial organs are very similar to those in species of Typhlopolycystis ( Figs 1C and G View FIGURE 1 ). The hard parts of the copulatory organ ( Figs 1G–I View FIGURE 1 ) consist of a gutter-like main stylet (prostate stylet) that receives the prostate glands and the seminal duct (stylet and prostate glands type III); and an accessory stylet that receives the duct of the accessory secretion (accessory stylet and accessory vesicle type II). The prostate stylet is curved, with its concave side forming the ‘closed side’ of the curve. In several species, this side is noticeably thickened. The tubiform and needle-like accessory stylet emerges from the left ‘wall’ of the gutter (the stylet seen from its concave side) and passes along the right side towards the concave side of the main stylet. In some species, such as L. mutabilis sp. nov., the accessory stylet can be very small ( Fig. 7 View FIGURE 7 ), while in others, such as L. conglobata sp. nov., it can be nearly as long as the main stylet ( Fig. 6 View FIGURE 6 ).

In most species of Typhlopolycystis , the accessory stylet is attached to the main stylet by a ring, and in species of the ‘ rubra group’, this ring is very narrow, and the accessory stylet is located next to, but not attached to, the main stylet (see Schockaert et al., 2019). Therefore, the difference with the construction of the hard parts of the copulatory organ of the species of Lagenopolycystis is not always clear.

The presence of a sperm-filled seminal receptacle in species of Typhlopolycystis or a sclerotised tube in species of Lagenopolycystis near the ovary remains a good distinctive character for both genera. As mentioned before, the tube is often not visible in species of Lagenopolycystis , but the seminal receptacle in species of Typhlopolycystis is evident, especially in live specimens.

Taxonomic account

Genus Lagenopolycystis Artois & Schockaert, 2000 View in CoL

Emended diagnosis after Brunet (1965). Species of Typhlopolycystidinae with a proboscis of 1/5 to 1/4 of the body length and with eyes. Gonads unpaired. Copulatory organ with prostate vesicle type III and accessory vesicle type II, a gutter-like prostate stylet type III, and a tubiform accessory stylet type II, the latter attached to one of the walls of the prostate stylet. With a sclerotised tubiform appendage (‘seminal receptacle’) at the entrance of the oviduct.

Type species. Lagenopolycystis peresi ( Brunet, 1965) Artois & Schockaert, 2000 View in CoL by original monotypy.