Page:EB1911 - Volume 12.djvu/958

Rh HAPLODRILI (so called by Lankester), often called Archiannelida (Hatschek), the name provisionally given to a number of interesting lowly-organized marine worms, whose affinities are very doubtful (see .) Polygordius and Protodrilus live in sand, but while the former moves by means of the contraction of its body-wall muscles, Protodrilus can progress by the action of the bands of cilia surrounding its segments, and of the longitudinal ciliated ventral groove. Saccocirrus, which also lives in sand, and more closely resembles the Polychaeta, has throughout the greater length of its body on each segment a pair of small uniramous parapodia bearing a bunch of simple setae. No other member of the group is known to have any trace of setae or parapodia at any stage of development.

These three genera have the following characters in common. The body is composed of a large number of segments; the prostomium bears a pair of tentacles; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed, the septa are distinct, and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom. Polygordius differs from Protodrilus and Saccocirrus in the absence of a distinct suboesophageal muscular pouch, and in the absence of a peculiar closed cavity in the head region, which is especially well developed in Saccocirrus, and probably represents the specialized coelom of the first segment. Moreover, in Saccocirrus the genital organs, present in the majority of the trunk segments, have become much complicated (fig. 2). In the female there is in every fertile segment a pair of spermathecae opening at the nephridiopores. In the male there are a right and a left protrusible penis in every genital segment, into which opens the nephridium and a sperm-sac. The wide funnels of the nephridia of this region are possibly of coelomic origin.

Dinophilus is a free-swimming form without tentacles, and with segmental bands of cilia (fig. 1). The parasitic Histriodritus (Histriobdella) feeds on the eggs of the lobster. It resembles Dinophilus in the possession of a ventral pharyngeal pouch (which bears teeth in Histriodrilus only), the small number of segments, and absence of distinct septa, the absence of a vascular system, the presence of distinct ganglia on the ventral nerve cords, and of small nephridia which do not appear to open internally. Histriodrilus resembles Saccocirrus in the possession of two posterior adhesive processes, and to some extent in the structure of the complex genital organs, which, however, are restricted to a single segment. In Dinophilus, there is also only a single pair of genital ducts behind; and in the male there are sperm-sacs and a median penis. In some species of Dinophilus there is pronounced sexual dimorphism (the male being small and without gut) as in the Rotifera. The resemblance of Dinophilus to the Rotifera is, however, quite superficial, and the general structure of this genus with distinct traces of segmentation, especially in the embryo, points to its close affinity, if not to Polygordius in particular, at all events to the Annelida.

That Polygordius, Protodrilus and Saccocirrus are on the whole primitive forms, and related to each other, there can be little doubt, but their place amongst the Annelida is difficult to determine. The development of Polygordius alone is well known, having been studied by Hatschek, Fraipont and others. The larva (fig. 1, C and D) is a typical but very specialized form of trochophore, provided with a branching nephridium bearing solenocytes. The trunk develops on the lower surface of the disk-like larva, which undergoes a more or less sudden metamorphosis into the young worm (fig. 1). There appears to be little either in the development or in the structure of the Haplodrili to warrant the view held by Hatschek and Fraipont that Polygordius and Protodrilus are exceedingly primitive forms, ancestral to the whole group of seta-bearing Annelids (Oligochaeta, Polychaeta, Hirudinea and Echiuroidea). 