Haplodrili, or Archiannelida, is a primitive marine worm part of the annelid phylum. Zoologist Ray Lankester gave it its name of Haplodrili, while zoologist Berthold Hatschek named it Archiannelida.


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 small and resembles polychaete larvae, epidermis ciliated, the number of segments varies from five and up or can be completely absent, small prostomium with or without appendages, parapodia absent, septa reduced or absent, the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (withotit distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed 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 of a transverse section of Saccocirrus showing on the left side the organs in a genital segment of a male, and on the right side the organs in a genital segment of a female.


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