The middle embryonic germ layer, which forms only in bilaterally symmetrical animals, is called the mesoderm.
Missing Word
mesoderm
The mesoderm is the germ layer between ectoderm and endoderm in triploblastic animals. It gives rise to tissues such as muscle and, in coelomates, the lining of the body cavity.
Category: Body Plans and Phylogeny
Question 2
Animals such as flatworms that have no body cavity between the gut and the body wall are called acoelomates.
Missing Word
acoelomates
Acoelomates have no fluid-filled body cavity; the space between the gut and the body wall is filled with tissue derived from mesoderm. Lacking a body cavity, flatworms rely on diffusion and stay thin.
Category: Body Plans and Phylogeny
Question 3
Freshwater sponges reproduce asexually by forming gemmules, clusters of cells enclosed in a tough outer coat.
Missing Word
gemmules
Gemmules are resistant packets of sponge cells that can survive harsh conditions such as winter and later grow into new sponges. Sponges also reproduce asexually by fragmentation and budding.
Category: Sponges and Cnidarians
Question 4
The jelly-like layer between the outer and inner cell layers of a sponge is called the mesohyl.
Missing Word
mesohyl
The mesohyl separates a sponge's outer flattened cells from its inner layer of choanocytes. Amoebocytes travel through it, carrying nutrients and building spicules.
Category: Sponges and Cnidarians
Question 5
The bell-shaped, free-swimming body form of a cnidarian, with its mouth and tentacles hanging downward, is called a medusa.
Missing Word
medusa
A medusa is the motile, bell-shaped body form seen in jellies. The other cnidarian body form is the sessile polyp, whose mouth and tentacles face upward.
Category: Sponges and Cnidarians
Question 6
Free-living freshwater flatworms known for regenerating a whole body from small fragments are called planarians.
Missing Word
planarians
Planarians belong to the flatworm class Turbellaria and glide on a ciliated underside. Many species can regrow a complete animal from a small piece of the body.
Category: Worms and Arthropods
Question 7
Each repeating body segment of a tapeworm, containing its own set of reproductive organs, is called a proglottid.
Missing Word
proglottid
A proglottid is one of the many segments that make up the long body of a tapeworm behind its head. Each holds reproductive structures, so a single tapeworm can release huge numbers of eggs.
Category: Worms and Arthropods
Question 8
The first pair of appendages in spiders, which serve as mouthparts and are often modified to inject venom, are called chelicerae.
Missing Word
chelicerae
Chelicerae are the specialized feeding appendages that give the subphylum Chelicerata its name. In spiders they end in fangs that deliver venom.
Category: Worms and Arthropods
Question 9
The small openings on an insect's body surface that let air into its tracheal tubes are called spiracles.
Missing Word
spiracles
Spiracles are pores in the exoskeleton that open into the branching tracheal system, which carries air directly to the tissues.
Category: Worms and Arthropods
Question 10
The dorsal flap of tissue in mollusks that can secrete a calcium carbonate shell is called the mantle.
Missing Word
mantle
The mantle covers the visceral mass and encloses the mantle cavity, which holds the gills and other structures. In shelled mollusks it secretes the shell.
Category: Mollusks and Annelids
Question 11
The paired fleshy appendages that bear many bristles on each segment of a polychaete worm are called parapodia.
Missing Word
parapodia
Parapodia are lobe-like extensions on the sides of polychaete segments that carry numerous bristles and help with movement. Oligochaetes such as earthworms lack them.
Category: Mollusks and Annelids
Question 12
The sieve-like plate on the upper surface of a sea star that regulates the amount of water in its water vascular system is the madreporite.
Missing Word
madreporite
The madreporite is the opening through which water enters and is regulated in the echinoderm water vascular system, which powers the tube feet.
Category: Echinoderms and Chordates
Question 13
The flexible supporting rod that runs between the nerve cord and the digestive tube in every chordate embryo is the notochord.
Missing Word
notochord
The notochord is one of the four defining chordate features. In vertebrates it helps induce the neural tube in the embryo and is later replaced by the vertebral column.
Category: Echinoderms and Chordates
Question 14
In bony fishes, the protective bony flap that covers the gills is called the operculum.
Missing Word
operculum
The operculum shields the delicate gills of bony fishes and helps move water over them. Sharks and rays lack an operculum and have exposed gill slits instead.
Category: Vertebrates
Question 15
Frogs and toads make up the amphibian order Anura, whose name means without a tail.
Missing Word
Anura
Anura is the frog and toad order of amphibians, named for the tailless adults. The other amphibian groups are the salamanders and newts (Urodela) and the limbless caecilians (Apoda).
Category: Vertebrates