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Jetting in jellyfish, scallops, squids leads on to: . Lecture 9 Bio 325. Jetting propulsion in Arthropoda, Odonata: whichfunctions also with gas exchange. Jetting propulsion in colonial Hydrozoa involves nectophores. Jetting propulsion by salps: tunicates in chains.

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Lecture 9 bio 325

Jetting in jellyfish, scallops, squids leads on to:

Lecture 9 Bio 325

Jetting propulsion in Arthropoda, Odonata: whichfunctions also with gas exchange.

Jetting propulsion in colonial Hydrozoa involves nectophores.

Jetting propulsion by salps: tunicates in chains.

Notochord as muscular hydrostat


Freshwater insects that use jetting to escape and to breathe.

Some dragonfly immatures (Order Odonata) jet water out of their rectum, the posterior chamber of their gut which is also a respiratory chamber; they use nozzle-shaped (tapered) terminal sclerites to increase momentum (mass X velocity) and to aim flow just like a squid siphon; telescoping in and out of abdominal segments + haemocoel pressure powers water intake and outflow; filaments filled with tracheae project into rectum lumen for gas exchange.

Mill P.J., Pickard R.S. 1975. Jet-propulsion in anisopteran dragonfly larvae. J. comp. Physiol. 97(4): 329-338.

Velella by the wind sailor using the wind for locomotion
Velella breathe.: By-the-wind sailor: using the wind for locomotion

Wilson, E.O. 1975. Sociobiology. Harvard, Cambridge Mass.


from Meglitsch P.A. Invertebrate Zoology

gastrozooid, gonozooid, dactylozooid, pneumatophore, etc.

Physalia: breathe.Portuguese Man ‘o War




They use stinging cells nematocysts

to capture fish prey.

Zooids: nectophores: squirt out jets of seawater to propel the colony.

gastrozooids are sac-like, specialized for ingestion and distribution of nutrients to rest of colony dactylozooids with batteries of nematocysts etc.




Nectophore zooids in a hydrozoan Cnidarian breathe.

A colony of zooids: at one end specialized as swimming individuals called nectophores, also called swimming bells. They jet seawater out of their subumbrellar openings, moving the colony, trailing behind on a fishing stem (stem transports nourishment by a shared coelenteron) about in the water column, presumably in a co-ordinated fashion.

National Geographic

Kevin Raskoff

Tunicate urochordata
Tunicate: Urochordata breathe.

  • Sea squirts are small barrel-shaped creatures often in clusters. They have a ‘tadpole larva’ dispersal phase that has a notochord (one of the reasons to classify them as Chordata), but it is not present in adults. An incurrent siphon, see terminal os above, brings seawater into a slitted pharynx which filters food; excurrent exits at atsthrough 2nd siphon.

  • Salp: breathe.semitransparent cylindrical free-swimming tunicate :Cyclosalpa affinis.

  • Phylum Chordata, Subphylum Vertebrata. Another chordate subphylum is Cephalochordata (amphioxus/Branchiostoma).Phylum Chordata, SubphylumUrochordata: Tunicates are sessile ‘sea squirts’ with a tadpole larval dispersal stage: characterized by 1) notochord 2) dorsal hollow nerve cord 3) pharyngeal clefts related to filter feeding. Some tunicates are pelagic: salps.

“A propulsive jet for locomotion is created by rhythmic compression of muscle bands encircling the barrel-shaped body. Fluid enters the anterior oral siphon to fill the mostly hollow body of the salp. ...oral lips close and circular muscle bands contract, decreasing the volume of the jet chamber so that fluid is accelerated out of the posterior atrial siphon.” [antagonists?] “...unique in possessing incurrent and excurrent siphons on opposite ends of the body allowing for unidirectional flow and reverse swimming during escape”.


Peter J. Bryant

Salp chains individuals zooids strung together in colonies

Colonial tunicates breathe.

Salp chains: individuals (zooids) strung together in colonies

Assigned reading:

Sutherland K.R., Madin L.P. 2010. Comparative jet wake structure and swimming performance of salps. J. exp. Biol. 213: 2967-2975.

Kenneth Kopp

Sutherland fig 3
Sutherland Fig. 3 breathe.

Are there fish

evolved to jet?

Chordata and the notochord as a breathe.hydrostatic* axial skeleton*Kier mentions notochord under ‘Additional examples’

  • Name of the phylum – Chordata** -- comes from notochord; chordates are diagnosed by certain body features shared by all species in the phylum:

  • pharyngeal gill slits: a perforated pharynx

  • dorsal tubular (hollow) nerve cord (dorsal to the digestive tract)***

  • tail, body continues postanal (anus is not terminal)

  • circulation occurs from the heart forward in a ventral vessel and then up around the pharynx and rearward in a dorsal vessel***

  • jointed endoskeleton

  • --- and at some time in their development all chordates have a notochord, a long cylindrical tube bounded by helical connective tissue fibres. surrounding a core of cells and fluid; this is also functioning as a hydrostatic structure.

***contrast with annelids: ventral solid nerve cord; anterior blood flow in dorsal vessel

Chordata subphylum cephalochordata
Chordata, Subphylum Cephalochordata breathe.

myotome block separations

evident here just below dorsal ‘fin’

Morgan Mccomb

Barrington E.J.W. 1965. The Biology of the Hemichordata and Protochordata.

Oliver & Boyd, Edinburgh & London.

IASZoology.com breathe.

  • There are ‘fins’, very different than the mobile bone-reinforced fins of fishes.

  • A pair of metapleural folds run ventrally, right and left, back to the atriopore (this being a separate excurrent exit for filtered seawater*).

  • These longitudinally alligned structures should lend stability against shear forces involved in rolling (canoes without keels are liable to roll).

  • The notochord is located just below the nerve chord, an intimate structural relationship because the the nervous system co-ordinates body side-to-side bending via the notochord.

  • * Filter feeding is accomplished by the slitted pharynx with micro-organisms separated from an incurrent created by beating cilia..


Assigned reading: Long, J.H. Jr. et al. 2002. The notochord of hagfish Myxine glutinosa: visco-elastic properties and mechanical functions during steady swimming. J. exp. Biol. 205: 3819-3831.

  • “Chordates have evolved an unique hydrostatic axial skeleton, the notochord, that is present in all taxa of that phylum early in development; it is retained in the adults of some taxa and modified by vertebral elements in others... Notochords are hypothesized to have evolved to stiffen the body (Goodrich, 1930) and to prevent body compression during muscle activation (Clark, 1964). In addition, notochords may adjust function by means of dynamically variable mechanical properties (Long et al., 1998).

Notochord is another example of a hydrostat, and given that it involves muscle,

perhaps it is reasonable to think of it as a muscular hydrostat.

Cell structure of notochord
Cell structure of notochord of hagfish


  • Muscles of amphioxus are arranged in a series of V-shaped blocks known as myotomes, separated by sheets of connective tissue, myocommas. Because they are v-shaped several appear in any single transverse section.

  • The muscle fibres within these v-shaped myotomes run longitudinally (as is also the case with fish).

All quotes from older Kardong 2 blocks known as nd edition 1998(Kardong 6th edition 2012, is on reserve) and must deal with notochord

  • “Such mechanical structures, in which the outer wall encloses a fluid core, are called hydrostatic organs.”

  • “The notochord is a hydrostatic organ with elastic properties that resist axial compression.”

  • Kardong does an ‘IMAGINE IT AS IT ISN’T’

  • “To understand the notochord’s mechanics, imagine what would occur if one block of muscle contracted on one side of an animal without a notochord. As the muscle shortens it shortens the body wall of which it is a part and telescopes the body [animal collapses longitudinally like an accordion] . In a body with a notochord, the longitudinally incompressible cord resists the tendency of a contracting muscle to shorten the body. Instead of shortening the body, the contraction of the muscle sweeps the tail to the side.” That is, the notochord functions to enable undulatory (rear-directed, i.e., retrograde, body waves). [plastic ruler illustration]

Summarizing the working and functions of the notochord: it is not just a simple fluid-filled chamber surrounded by connective tissue acting to translocate muscle forces

  • It is a hydrostat and at least in part a muscular hydrostat -- because a tunic of helical connective tissue fibres works against muscle and fluid within.

  • It stiffens the body longitudinally, preventing it from shortening when axial muscles contract.

  • It makes the longitudinally alligned muscle fibres of the right and left sides antagonists.

  • It promotes lateral (transverse) body bending, which is the basis of undulatory body waves.

  • Since it is composed in part of muscle cells (capable of contracting) there is a dynamic quality to its function as a skeleton: it can change its stiffness topographically to facilitate adaptive bending

Buoyancy is not just a simple fluid-filled chamber surrounded by connective tissue acting to translocate muscle forces

  • Density is mass per unit volume; so regulating your density is a matter of losing weight or increasing your volume. Some swimming animals regulate body density.

  • You soon realize as a scuba diver that swimming is much easier when you are not working to stay down or to keep from sinking, when you are neutrally buoyant.

  • A diver achieves neutral buoyancy by regulating body density with a buoyancy compensator (BC). Bony fishes do the same thing.

Buoyancy see vogel comparative biomechanics p 96
Buoyancy see Vogel is not just a simple fluid-filled chamber surrounded by connective tissue acting to translocate muscle forcesComparative Biomechanics p. 96

  • Archimedes’ Law : objects heavier than the volume of water they displace will sink; objects lighter than the volume of water they displace will rise. A fish is buoyed up by a force equal to the weight of the water it displaces. It can change this force by changing its volume, i.e., displacing more or less water. Secreting oxygen gas into its swim bladder from the blood, the fish increases its volume and displaces more water, so increasing the force acting to make it rise in the water column. Conversely it can absorb oxygen gas from the bladder and so sink.

Inland fishes of NY, Cornell

Swim bladder gas bladder
Swim bladder /Gas bladder is not just a simple fluid-filled chamber surrounded by connective tissue acting to translocate muscle forces

Many bony fishes have a single median gas bag in their body used to change their density, giving neutral buoyancy at different levels in the water column. This bladder, situated just below the backbone and just above the viscera, contains oxygen at a high concentration; the oxygen is actively secreted from the blood.

Ancestors of bony fishes, living in fresh water, evolved lungs to supplement their gills in times of drought. When some of these ancestors reinvaded the seas these lungs evolved into swim bladders.

Fisheries & Oceans Canada