Phylum Gastrotricha
Phylum
Gastrotricha
Gastrotricha (gas-tro-tri´ka) (N. L. fr. Gr., gaster, gastros, stomach or belly, + thrix, trichos, hair) includes small, ventrally flattened animals about 65 to 500 µm long, somewhat like rotifers but lacking a corona and mastax and having a characteristically bristly or scaly body. They are usually found gliding on the bottom, or on an aquatic plant or animal substrate, by means of their ventral cilia, or they compose part of the meiofauna in interstitial spaces between bottom particles.
Gastrotrichs are found in both fresh and salt water. The 400 or so species are about equally divided between the two media. Many species are cosmopolitan, but only a few occur in both fresh water and the sea. Much is yet to be learned about their distribution.
Form and Function
A gastrotrich (Figures 15-5 and 15-6) is usually elongated, with a convex dorsal surface bearing a pattern of bristles, spines, or scales, and a flattened ciliated ventral surface. Cells on the ventral surface may be monociliated or multiciliated. The head is often lobed and ciliated, and the tail end may be forked.
A syncytial epidermis is found beneath the cuticle. Longitudinal muscles are better
developed than are circular
ones, and in most cases they are
unstriated. Adhesive tubes secrete a
substance for attachment. A dual-gland
system for attachment and release is present, similar to that described for
Turbellaria. The pseudocoel is
somewhat reduced and contains no
amebocytes.
The digestive system is complete and is made up of a mouth, a muscular pharynx, a stomach-intestine, and an anus (Figure 15-5B). Food is largely algae, protozoa, and detritus, which are directed to the mouth by the head cilia. Digestion appears to be extracellular. Protonephridia are equipped with solenocytes rather than flame cells. Solenocytes have a single flagellum enclosed in a cylinder of cytoplasmic rods.
The nervous system includes a brain near the pharynx and a pair of lateral nerve trunks. Sensory structures are similar to those in rotifers, except that eyespots are generally lacking. Sensory bristles, often concentrated on the head, are modified from cilia.
Gastrotrichs are hermaphroditic, although the male system of some is so rudimentary that they are functionally parthenogenetic females. Like rotifers, some gastrotrichs produce thin-walled, rapidly developing eggs and thickshelled, dormant eggs. The thickshelled eggs can withstand harsh environmental conditions and may survive dormancy for some years. Development is direct, and juveniles have the same form as adults.
 |
| Figure 15-5 Chaetonotus, a gastrotrich. A, Dorsal surface. B, Internal structure, ventral view. |
Gastrotricha (gas-tro-tri´ka) (N. L. fr. Gr., gaster, gastros, stomach or belly, + thrix, trichos, hair) includes small, ventrally flattened animals about 65 to 500 µm long, somewhat like rotifers but lacking a corona and mastax and having a characteristically bristly or scaly body. They are usually found gliding on the bottom, or on an aquatic plant or animal substrate, by means of their ventral cilia, or they compose part of the meiofauna in interstitial spaces between bottom particles.
Gastrotrichs are found in both fresh and salt water. The 400 or so species are about equally divided between the two media. Many species are cosmopolitan, but only a few occur in both fresh water and the sea. Much is yet to be learned about their distribution.
Form and Function
A gastrotrich (Figures 15-5 and 15-6) is usually elongated, with a convex dorsal surface bearing a pattern of bristles, spines, or scales, and a flattened ciliated ventral surface. Cells on the ventral surface may be monociliated or multiciliated. The head is often lobed and ciliated, and the tail end may be forked.
A syncytial epidermis is found beneath the cuticle. Longitudinal muscles are better
 |
| Figure 15-6 Gastrotrichs in order Macrodasyida, A, Macrodasys. B, Turbanella. |
The digestive system is complete and is made up of a mouth, a muscular pharynx, a stomach-intestine, and an anus (Figure 15-5B). Food is largely algae, protozoa, and detritus, which are directed to the mouth by the head cilia. Digestion appears to be extracellular. Protonephridia are equipped with solenocytes rather than flame cells. Solenocytes have a single flagellum enclosed in a cylinder of cytoplasmic rods.
The nervous system includes a brain near the pharynx and a pair of lateral nerve trunks. Sensory structures are similar to those in rotifers, except that eyespots are generally lacking. Sensory bristles, often concentrated on the head, are modified from cilia.
Gastrotrichs are hermaphroditic, although the male system of some is so rudimentary that they are functionally parthenogenetic females. Like rotifers, some gastrotrichs produce thin-walled, rapidly developing eggs and thickshelled, dormant eggs. The thickshelled eggs can withstand harsh environmental conditions and may survive dormancy for some years. Development is direct, and juveniles have the same form as adults.
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