One view of the origin of the latter (largely based upon observations upon the development of Polygordius) sees in the blood system a persistent blastocoel.
They are repeated in Polygordius, and are exactly Cs.
8, D) (which has a male of the same type as Melicerta, &c.) is an extremely modified type, and its resemblance to the trochophore larva of Lepadorhynchus or Polygordius is only superficial.
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.
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.