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Pseudocoelomates (“Gnathiferans”)ZOO 2040 Biology of Animals Topic 7

Pseudocoelom■ Original hollow center (blastocoel)!

of embryo persists as a space or !cavity between the enteron and!body wall:• May be filled with fluid or gelatinous!

material with mesenchyme cells.• Provides greater freedom of !

movement (structural support is!provided by the hydrostatic!pressure of the fluid in the!pseudocoelom against the body !wall, and the cavity is a base for !muscles used in locomotion).• Provides greater regulation of internal!

environment and more space for !digestive, excretory and reproductive!systems.• Helps distribute materials through!

the body. Provides a storage area for waste products.■ Incompletely lined with mesoderm: more complex animals have a true

coelom formed from mesoderm and lined with peritoneum.

Pseudocoelomates■ Polyphyletic: more than one acoelomate!

ancestor. Numerous classification schemes.■ Of the ten phyla, six (Gnathostomulida,!

Rotifera, Gastrotricha, Kinorhyncha,!Nematoda, and Nematomorpha) have!similarities which once caused them to be!included in a single phylum (Aschelminthes)

■ Entoprocta and Ectoprocta were once!grouped into a single phylum (Bryozoa), but!ectoprocts have a true coelom.

■ All have a body wall that is often syncytial,!a dermis, and muscles surrounding the!pseudocoel.

■ Digestive tract, excretory organs, and gonads are within pseudocoel, bathed in perivisceral fluid. Digestive tract is one-way (except for the parasitic Acanthocephala).

■  In some groups, the epidermis secretes an acellular (‘non-living’) cuticle.■ Most are dioecious (gonochoristic): separate males and females.■ Many are eutelic (each species has a constant number of cells or cell nuclei);

growth is by cellular enlargement, rather than by cell proliferation.

Phylum Rotifera !(“wheel bearers”)■  About 2000 species, 95%

freshwater. Very diverse, most between 100-500 µm (range: 40 µm-3 mm). 3 body regions: head, trunk, and tail (with a foot having pedal glands and 1-4 toes for attachment). ‘Floaters’ (usually globular or sac-like), ‘creepers’ and ‘swimmers’ are more elongate. Sessile forms are vase-like, with thick epidermis (lorica). Some are colonial. Some are parasites.

■  Large pseudocoel. Syncytial and eutelic. Most have very constant numbers of cell nuclei in each organ and very precise control of cell division and differentiation.

Phylum Rotifera■ Two ciliated anterior

trochal disks form a corona (used for locomotion and feeding). Pharynx (mastax) has muscular portion with hard ‘jaws’ (trophi) for sucking in/grinding up food. Feed on protozoa and small animals. Salivary, gastric glands secrete digestive enzymes. Absorption in stomach.

■ Excretion by a pair of protonephridial tubules (each w/ several flame cells). Bladder, intestine, and oviducts empty into a cloaca.

■ Bilobed brain dorsal to mastax. Eyespots and other sensory organs.

Phylum Rotifera (cont.)■ Many rotifers show cyclomorphosis (morphological

changes in subsequent generations).■ Response to seasonal variation in:• Food type.• Dissolved organic substances,!

including presence of chemicals!that signal presence of predators.• Rotifers cultured in water !

containing chemical extracts of !predators increased both body size!and spine length, which significantly !reduced their predation risk.• Not due to genetic differences: all!

experimental populations were!parthenogenetic ‘clones’ of the!same female.

Phylum Rotifera (cont.)■ Dioecious. Size dimorphism (males smaller).■ Bdelloidea females are parthenogenetic (produce!

diploid eggs that hatch into diploid females).■ Monogononta also typically parthenogenetic• Females produce diploid amictic (without ‘mixing !

genes’) eggs by mitosis (usually 1 at a time) that hatch!into diploid females.• Food quality/quantity, photoperiod, and temperature!

influence production of haploid mictic eggs (generally !only 1 or 2 generations/year) by meiosis, if unfertilized,!these become haploid, non-feeding males: —  Males thus often found only during brief periods!

(can mate about 1 h after hatching).—  Have a single testis and ciliated sperm duct running !

to a genital pore and copulatory organ.—  Mating by hypodermic penetration (sperm injected into female pseudocoel).

Fertilized eggs can overwinter as resting (winter) eggs.• Small number (<0.5%) are amphoteric (produce both haploid and diploid eggs)

■ Female reproductive system may provide yolk to developing ova by cytoplasmic bridges, rather than separate yolk cells of acoelomates

■ Ovaries and yolk glands combined into single structures (germovitallaria)

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Pseudocoelomates (“Gnathiferans”)ZOO 2040 Biology of Animals Topic 7

■  Less than 1,200 species, all vertebrate gut parasites:!mostly freshwater fishes (important in aquaculture,!but also amphibians, reptiles, birds and mammals).!About 2 mm to 1 m). Related to rotifers.

■  Name comes from spiny, eversible and retractable!proboscis (used to attach and pull body close to !gut wall), can cause damage.

■  Body wall comprises thin cuticle over a syncytial epidermis (nutrient absorption probably occurs across cuticle). Epidermis contains unique system of channels (lacunar system) used for circulation.

■  All vestiges of digestive system !and protonephridia absent!except in a few species!(being parasites, all non-!gonadal organ systems are!reduced)

■  Unique set of ligaments!and ligament sacs!partially partitioning !the body into one!or two large sacs lined!with connective tissue!(ligament sacs) that house the reproductive organs.

Phylum Acanthocephala (“spine- or thorn-headed”) Phylum Acanthocephala (cont.)■ Dioecious. Males have cement glands, used to plug female!

gonopore after mating.■ Females have a complex!

structure called a uterine bell that!holds zygotes until they reach!the acanthor stage:• Actually sorts the embryos and only lets mature larvae out.• Acanthor released with feces, ingested by intermediate host, usually an insect or

crustacean (mollusk in 1 species)!and enters body cavity.• Prior to developing into an!

encysted cystacanth form,!acanthors may ‘hijack’ the host’s!nervous system, altering its!behavior to improve the odds of !transmission to definitive host,!e.g., Plagiorhynchus cylindraceus!disrupts light avoidance behavior !of the terrestrial isopod (pillbug)!Armadillidium vulgare.• Cystacanth ingested by definitive!

host, reaches sexual maturity.

Phylum Gastrotricha!(“belly hair”)■ Small (about 65-500 µm, rarely 1!

mm long), dorsoventrally !flattened, usually elongate. Bristly,!spiny, or scaly body. Head w/!numerous sensory bristles.

■ Freshwater and marine. Usually !glide on bottom, or on plant or !animal surfaces, using ventral!cilia, or live interstitially (up to 100,000/m2). Adhesive tubes secrete attachment substances (dual-gland system resembles turbellarians).

■ Feeders on small particles (detritus, bacteria, diatoms, and protozoans). One-way extracellular digestive system.

■ Like rotifers but lack corona, mastax, and eyespots. Unlike rotifers, they are mostly hermaphroditic (many freshwater forms are parthenogenic). Internal fertilization and determinant embryo cleavage (but cleavage pattern is bilateral and radial, not spiral). No free-living larvae. Eggs may be thick-shelled and remain dormant for long periods.

■ Pseudocoelomate status very questionable (probably acoelomate). About 800 species.

Phylum Gnathostomulida (“jaw-mouths”)■ Delicate, vermiform, 0.5-1 mm long. Pharynx has!

a pair of lateral ‘jaws’ and an associated basal plate!to scrape bacteria and fungi from substrates.

■ First observed in 1928 in Baltic Sea sediments, but!not described until 1956. Found in ME, FL Keys, the!Caribbean, N. CA, the Indo-Pacific, and White Sea.

■ Form part of the infauna (small animals that live!between grains of marine sediments and silt), at <0.5!mm, they are also part of the meiofauna:• Tolerate low oxygen, high H2S substrates.• Often very abundant (as many as 6,000/kg sand),

■ Triploblastic: have a mouth, but no anus.■ Lack a pseudocoelom, and like acoelomates, they !

are externally ciliated.• Only one cilium per cell, unlike the several cilia per cell of !

flatworms: resembles planula larva of cnidarians.■ No circulatory or respiratory systems; protonephridial !

excretory system. Most are hermaphroditic; internal!fertilization of eggs, protostome (spirally cleaving) embryo; no larvae (adults develop directly from embryos). About 80 spp., 18 genera

Phylum Cycliophora !(“wheel bearer”)■ Recently-described (1995) animal!

phylum: 1 species (Symbion pandora):• <500 µm long. Attach by an adhesive disc!

(1) at the base of a short stalk.• Live exclusively as commensals on the!

mouth appendages of lobsters, feeding !on particles shredded and dropped as the!lobster eats.

■ A ring of compound cilia (4) encircles!the mouth; works very similarly to the!rotifer corona. Anus (3) is located in the ‘neck’, outside of this funnel.

■ U-shaped digestive tract (similar to lophophorates), lined with multiciliated cells (rotifers also have multiciliated epithelial cells)

■ Very strange and complex life cycle; note the non-feeding dwarf male (2) attached to the outside of the body of the female.

■ 18S sequence indicates relationship to rotifers and acanthocephalans:• Rotifers also have smaller males, hypodermic injection of sperm, and multiciliated

epithelial cells. Body is covered by a thin cuticle, which also resembles that of some pseudocoelomates.

■ Modular growth by budding, some aspects of brooding young, and periodic regeneration of feeding apparatus, are all similar to entoprocts.

I didn’t even know that lobsters had lips, but it turns out that they do, and these lips are the stomping ground of a tiny creature called Symbion pandora (literally, a ‘couple of Greek words’) -Dave Barry

Phylum Micrognathozoa!(“micro-jawed animal”)■ Most recently-described (2000) animal!

phylum: 1 species (Limnognathia!maerski).

■ Discovered in a cold freshwater spring !near Isunngua (eastern Disko Island, Greenland)!during a field course:• Lives within mosses in the spring: uses ventral cilia to either !

swim or glide slowly on the moss. Eats bacteria, cyanobacteria,!and diatoms, the cilia on the head sweep food particles towards!the mouth where they are grabbed by the ventral jaws.

■ The spring at is frozen from October to May in most years,!and during the short summer the average temperature is!about 8 °C. Adults cannot survive the winter, so must eat!and reproduce in the summer before the water freezes again. • Apparently reproduces mostly by parthenogenesis, but (like!

rotifers) produces two kinds of eggs: a thick-shelled, overwintering !egg and a more thin-shelled, summer egg. Thick-shelled eggs are only produced after male fertilization, so it has been proposed that L. maerski is also a dioecious protandrous hermaphrodite (individuals hatch as males, remain males for a short period, and then become females when they grow older).