Aquatic Invertebrates of Montana

Annelida

Introduction

Segmented worms. A major protostome, coelomate phylum with about 9000 species worldwide. Most species are marine or terrestrial. They have closed circulatory system, a large ventral nerve cord, and excretion by nephridia. They have a trochophore larvae during development, . The life cycle is non-emergent.

Recognition

Worms with a metameric body, often much large than other worms. Both circular and longitudinal muscles are present and the coelomic fluid functions as a hydrostatic skeleton. Six classes occur in freshwater.

• Class Polychaeta- 5300 species of primary marine annelids. Polychaets have paired lateral appendages (parapodia), and many setae. They are dioecious with external fertilization and a planktonic trochophore larva. Only about 10 species in 3 orders are secondarily freshwater in North America. The freshwater species are small and poorly known. They may be free-moving, tube-dwelling or interstitial. They occur mostly near the sea coasts.

   

• Class Aphanoneura- one genus, Aeolosoma of minute oligochaet-like worms, but with ventral hair setae, no clitellum, unfused nervous system, and they move by gliding using the ciliated prostomium.

   

• Class Oligochaeta- 3100 species of primary freshwater annelids, but most species are secondarily terrestrial, some are secondarily marine. They are hermaphroditic with internal fertilization and direct development. They lack parapodia and they have few setae.

   

• Class Brachiobdella- A holartic group with about 100 neartic species of leech-like primary freshwater annelids that are parasites or commensals in the gill chamber of freshwater crayfish. They are less than 10 mm in length, with 17 body segments, the mouth is encircled with finger-like projections and they have a posterior sucker.

   

• Class Hirudinea - leeches, 500 species (63 in North America) of primary freshwater annelids, some species are secondarily marine or terrestrial. Leeches lack parapodia and setae. They are hermaphroditic with internal fertilization and direct development. They have only 34 true segments, but these may be secondarily annulate. Leeches have anterior and posterior suckers.

   

• Class Acanthobdellida- A single species Acanthobdella peledina Grube, at least in the neartic region. This is a leech-like worm 10-22 mm long, but it has 2 pair of chaetae on 5 segments in the head region, no anterior sucker, 29 postoral segments and paired male gonopores. It is a temporary parasite on salmonid fishes in Alaska, Asia and Europe.

Taxonomy

Fairy well studied because of the limited size of the group and wide distribution of many species. The higher classification of many groups is still unsettled. Identification of the smaller species requires carefully prepared slide mounts and magnifications of up to 1000X. Primarily a specialist's group, but leeches should be readily identifiable by general aquatic workers.

Some changes since Pennak

• Aeolosomatidae (Aeolosoma) was removed from the Oligochaeta and placed in the new class Aphanoneura, which arose between the basal Polychaeta and the remaining 4 annelid classes, which are phlogenetically unresolved at this time.
• Archiannelida is now generally discarded as a class. Most of its species are interstitial and reduced Polychaeta.