Haplodrili, or Archiannelida, is an order of primitive polychaete worms.
Zoologist Ray Lankester gave it the name haplodrili, while zoologist
Berthold Hatschek
Berthold Hatschek (3 April 1854 in Skrbeň – 18 January 1941 in Vienna) was an Austrian zoologist remembered for embryological and morphological studies of invertebrates.
Life
He studied zoology in Vienna under Carl Claus (1835-1899), and i ...
later named it Archiannelida. Once considered to be a class under
Annelida, and even a separate phylum, Haplodrili is now widely accepted to be an order under
Polychaeta
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...
.
Species in this order are known for completely lacking external segments.
Overview
''
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.
Commonality
These three genera have the following characters in common. The
body
Body may refer to:
In science
* Physical body, an object in physics that represents a large amount, has mass or takes up space
* Body (biology), the physical material of an organism
* Body plan, the physical features shared by a group of animal ...
is small and resembles
polychaete
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ma ...
larvae; the epidermis is 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 (without distinct ganglia); this is widely separated in ''Saccocirrus'', closely approximated in ''Protodrilus'', and fused together in ''Polygordius''; the coelom is well developed and the dorsal and ventral longitudinal
mesenteries
In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms co ...
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 more complicated. In the female, every fertile segment has a pair of spermathecae opening at the nephridiopores. In the male, every genital segment has a right and a left protrusible penis, into which both the nephridium and a sperm-sac open. The wide funnels of the nephridia of this region are possibly of coelomic origin.
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See also
* Appendiculata
References
External links
Archiannelida (polychaete order)
Polychaetes
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