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Amphitrite ornata.

Amphitrite Ornata

Amphitrite ornata.

This illustration shows a transverse section of a worm with external gills. Gills are the breathing organs of aquatic animals.

Gills (worm)

This illustration shows a transverse section of a worm with external gills. Gills are the breathing…

Colonial, plant-like animals closely related to jellyfish, with stinging cells, Any member of the invertebrate order Hydroida (class Hydrozoa, phylum Cnidaria). Hydroids have three basic life-cycle stages: (1) a tiny free-swimming planula larva about 1 mm (0.04 inch) long, which settles and grows into (2) a sessile (attached), usually colonial hydroid stage, which in turn liberates (3) medusa.

Medusa of a Hydroid

Colonial, plant-like animals closely related to jellyfish, with stinging cells, Any member of the invertebrate…

"Diagram of Medusa, illustrating radial symmetry. A, viewed from the oral end of the principal axis; B, a section along the principal axis and through one of the subordinate axes aa1; o, ab. o, the oral and aboral poles of the principal axis; a, a1, and b, b1, the similar poles of the two chief subordinate axes." -Galloway, 1915

Medusa

"Diagram of Medusa, illustrating radial symmetry. A, viewed from the oral end of the principal axis;…

"Vertical section of a fresh-water sponge (Spongilla), showing the arrangement of the canal-system. C, ciliated chambers; DP, dermal pores; Ex, excurrent canals; GO, openings of the excurrent canals; PG, paragastric cavity; SD, subdermal cavities; O, osculum." -Parker, 1900

Spongilla

"Vertical section of a fresh-water sponge (Spongilla), showing the arrangement of the canal-system.…

Sycon gelatinosum, a sponge.

Sycon Gelatinosum

Sycon gelatinosum, a sponge.

"Transverse section through the wall of a cylinder (parallel with the course of the canals), showing one incurrent canal (IC), and one radial (R) throughout their length; sp, triradiate spicules; sp', oxeote spicules of dermal cortex (dc); sp'', tetraradiate spicules of gastral cortex (gc); ec, ectoderm; en, endoderm; pm, pore membrane; pp, prosopyles; ap, apopyle; di, diaphragm; exc, excurrent passage; PG, paragastric cavity; em, early embryo; em, late embryo. The arrows indicate the course of the water through the sponge." -Parker, 1900

Sycon Gelatinosum

"Transverse section through the wall of a cylinder (parallel with the course of the canals), showing…

A, Diagrammatic transverse section through the body of a vertebrate. B, A similar section through a highly organized invertebrate animal. Labels: a, neural canal; b, cerebrospinal nerve center; c, notochord; d, sympathetic nerve center; e, alimentary canal; f, visceral canal wall; g, haemal system.

Vertebrate and Invertebrate

A, Diagrammatic transverse section through the body of a vertebrate. B, A similar section through a…

"Young Balanoglossus: showing proboscis, collar, gill-slits, and gastric region. Balanoglossus, a worm-like animal of much zoological interest as a connecting link between invertebrates and vertebrates." -Hazeltine, 1894

Acorn Worm

"Young Balanoglossus: showing proboscis, collar, gill-slits, and gastric region. Balanoglossus, a worm-like…