Invertebrados

PHYLUM ANNELlDA 435
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(A)
Girdles or annuli
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(F)
(B)

(G)
Figure 13.38 Beard worms (Pogonophoridae). (A) A generalized pogonophoran. (B) The tube of Lamellisabella. (e) Anterior end of the mono-tentaculate Siboglinum in its tube. (D,E) Anterior end and tube of Polybrachia. (F) Enlarged view of the opisthosoma of Polybrachia. (G) Living specimens of the deep-sea, hydrothermal vent vestimentiferan Riftiapachyptila.
(C)

Septum between forepart and trunk

436 CHAPTER THIRTEEN
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Other Beard Worms (Birstenia, Polybrachia,
Siboglinum, etc.)
1. Anterior body cavities usually not well developed
2. Without vestimental wings
3. Opisthosoma with three nerve cords, all apparent-
Iy bearing ganglia
4. Opisthosomal coeloms lack medial mesenteries
5. Apparentlywithout gut at any time
6. Obturaculum absent
BOX 13B Some Characteristics of the Polychaete Family Pogonophoridae (the Bearded Worms)
Vestimentiferans (Lamellibranchia, Ridgeia,
Riftia, etc.)
1. Anterior body cavities well developed
2. Vestimental "wings" present
3. Opisthosoma with single, nonganglionated
nerve cord
4. Opisthosomal coeloms with medial mesenteries
5. Withtransitory gut in juveniles
6. Anterior obturaculum present
brown pigment rings (Figure 13.38B,C,E). The upper end of the tube projects aboye the substratum so the ten-taculate part of the worm can extend into the water.
The body surface is covered by a flexible cutiele that is thickened in various patterns, ineluding a collar-like ridge that apparently rests on the rim of the tube when theanimal is extended. The epidermis is mostly a cu-boidal to columnar epithelium and ineludes various gland cells, papillae, and ciliary tracts. Microvilli extend into the cutiele and produce part of its outer layer. Large glands extend inward from the epidermis, and sorne of the trunk epithelium is thought to contain absorptive cells. Beneath the epidermis is a thin layer of circular musele and athick layer of longitudinal musele, the latter developed as bands or bundles in sorne parts of the body.
The body cavities are fairly spacious except in the anterior end, where large mus ele bands occur. In at least sorne species, the cavity of the cephalic lobe in-eludes a small sacciform or U-shaped space with a pair of coelomoducts to the outside. The cavities of the forepart and trunk arepaired, with sagittally arranged mesenteries bisecting the body. (The cavities of these first three body regions were likened to the protocoel, mesocoel, and metacoel of deuterostomes until the body orientation was resolved and the opisthosoma was discovered.) The opisthosoma comprises several segments, each with cavities in a typical annelid-like metameric arrangement.
Nutrition
In the absenceof a functional digestive tract, the method of nutrition in many of these relatively large worms is indeed puzzling. Experimental work suggests that most beard worms are able to absorb dissolved organic mat-ter (DOM) from the sea water flowing across the tenta-eles and from the muddy sediments in which the ani-mals are buried. Apparently no digestive enzymes are involved in this process; ratherthe worms take in glu-
cose, amino acids, and fatty acids directly from the envi-ronment. Sorne species may also be capable of epider-mal pinocytosis and phagocytosis.
The absorption of organic nutrients probably also oc-curs in the giant worms associated with the chemoau-totrophic based ecosystems of hydrothermal vent envi-ronments. However, work on Siboglinum fiordicum indica tes that uptake...