The Systematic Position of the Branchiobdellidae (Annelida: Clitellata)

The annelid worms which are epizoic on holarctic freshwater crustaceans were considered by their discoverers to be leeches (Odier, 1823). At a later period they were assigned to the oligochaetes (Vejdovsky, 1884; Michaelsen, 1909; Pierantoni, 1912). The question of their affinities has recently been reopened (Franzen, 1963). I propose here to review the structure, distribution, and habits of these animals, currently included in the oligochaete family Branchiobdellidae, and to advance additional reasons for my previously expressed belief (Holt, 1963b) that this family should be elevated to ordinal rank. In the absence of a fossil record and with little hope of such a record ever being discovered, judgments of taxonomic rank for a group of animals must be based primarily on the comparative anatomy of generalized structural features. But the origin and subsequent radiation of a group has been influenced also by ecological factors and geographical relationships. Anatomy is fundamental and will be considered first, but other considerations should not be neglected. The body of the branchiobdellids is short and composed of 15 segments (see Stephenson, 1930, for review), of which the first four are set off as a head region (Fig. 1). There is no prostomium. The peristomium functions as an oral sucker and is set off from the remainder of the head by a sulcus. Each segment of the trunk region is subdivided in leech-like fashion by a constriction superficially similar to an intersegmental furrow into an anterior prosomite and a posterior metasomite. The last one or two segments are modified to form a sucker. The anus is borne on the dorsum of the thirteenth or fourteenth segment. In addition, the trunk carries the unpaired ventral openings of the spermatheca on the ninth segment, of the male efferent system on the tenth, and the paired openings of the oviducts latero-ventrally on the eleventh (Fig. 1). The nervous system is annelidan: the bilobed brain, circumpharyngeal connectives, paired ventral cords, and paired segmental ganglia, with some coalescence of ganglia anteriorly and posteriorly, are its components. The circulatory system consists of the usual annelidan dorsal vessel with four pairs of commissures connecting it with a ventral vessel in the head region and posterior vessels which return blood to the dorsal one. The excretory system consists of two pairs of nephridia, one pair located within the fifth through eighth segments, the other pair in the twelfth segment. The anterior pair opens by either paired pores or an unpaired common one on the dorsum of the seventh segment; the posterior nephridia open dorso-laterally, it is believed, on the thirteenth segment (Freeman, 1963). The digestive system is composed of a pharyngeal region in the head, an intestine which may be dilated in several of the segments, and a short narrowed portion just anterior to the anus. The intestine is invested with a peritoneally derived chloragogen and lacks caeca. The distinctiveness of the head as a separate body region is due primarily to the great development of the pharyngeal musculature. In all branchiobdellids the anterior part of the pharynx (in the second segment) is furnished with dorsal and ventral placoids (jaws) com-