Major Characters of Invertebrate Body Design

The idea of this list is to summarize some of the major trends in animal body design.  These are the characteristics that zoologists have been using for more than a century to organize their concepts of animal evolution, considering them to be reflections (if not firm evidence) of shared ancestry.   New, genetic information is challenging some of the old, embryological and anatomical, phylogenetic concepts, and it is increasingly plain that much real variation has for many years been glossed over in simplified schemes in an attempt to force animals into partly preconceived, superficially similar, "natural" groups.  For example, the "Aschelminthes" concept based on presence of a pseudocoelom has fallen apart as genetic evidence directed our attention away from such time-honored features as the nature of the body cavity, and toward such features as the presence of cilia, the shedding of the cuticle to grow, and the chemical nature of the cuticle.

Zoologists are still constrained by the fact that several different patterns among those listed below may occur within one otherwise very ccohesive group, and may be virtually identical across obviously different groups.  The second half of this problem is due to the common occurrence of ancestral and convergent similarities between ainimal taxa.

Nevertheless, students should be able to develop a clearer overview of animal diversity and evolutionary relationships by focusing on how each taxon that we study fits the criteria below.

Body symmetry

* asymmetric, radial, or bilateral
Symmetry is almost completely parallel to cell-tissue organization:
+ asymmetical ... cellular level of body organization, no tissues, Parazoa
+ radial ... diploblastic (two fundamental tissue layers), Radiata
+ bilateral ... triploblastic (three fundamental tissue layers), Bilateria

The phylum Mesozoa is bilateral but has two tissue layers (ectoderm and mesoderm?).
The phylum Ctenophora is usually considered to be radially symmetrical and diploblastic, but has bilateral features (e.g., tentacles, so it could be called bi-radial, as are the Anthozoa in Cnidaria), and distinct, internal muscle cells in the tentacles that look a lot like mesoderm.

Embryology (and Body Cavity Origin)

* radial, spiral, peripheral, or bilateral cleavage, or variously modified
+ protostome animals in the phylogenetic group Spiralia usually have spiral cleavage
but Bryozoa and other lophophorate spiralians have radial cleavage
+ deuterotstomes usually have radial cleavage
+ phyla in the phylogenetic group Ecdysozoa may have peripheral, bilateral, or modifed radial cleavage, or even (rarely) spiral cleavage

* cell fates after cleavage:

+ determinate cleavage (= mosaic; typical of most protostomes, embryonic cell fates are fixed from the first cleavage on) or
+ regulative cleavage (= indeterminate, typical of most deuterostomes, embryonic cells up to the third or later cleavage can regulate their fates to produce whole individuals)

* fate of blastopore:

+ anus (most deuterostomes), or
+ becomes the adult mouth (most protostomes) or both, or
+ closes over (e.g., echinoderms) before adult mouth and/or anus form de novo

* presence (pseudocoelomate or eucoelomate) or absence (acoelomate) of a body cavity between the digestive tract and the body wall.
* and if present, whether this cavity is ...

+ unlined (pseudocoelomate) and formed from the blastocoel,
+ lined (eucoelomate) and formed by splitting of mesodermal cell masses (schizocoelous), or
+ lined (eucoelomate) and formed by outpocketing of the endodermal archenteron (enterocoelous)
Evidence exists that some acoelomate animals evolved from eucoelomate animals with loss of the coelom (Platyhelminthes) or modification to form the rhynchocoel or circulatory system (Nemertea).

Many Ecdysozoa phyla are considered eucoelomate because an embryonic coelom is present in adults as a small cavity around or in the gonads, but the main body cavity is an unlined hemocoel.


Body Covering

* non-cellular, chemical cuticle entirely covers the exterior, and must be shed to grow (phylogenetic group Ecdysozoa), or + whether chitin is the dominant chemical in the cuticle (eucoelomate Ecdysozoa), or a minor component, or collagens (protein) are the dominant chemicals (most Ecdysozoa in the phylogenetic group Cycloneuralia).
+ cuticle is thick and strong enough to hold body shape and support major muscle attachments (exoskeleton; only in Arthropoda)), or thin and flexible
* living cells in contact with the external environment + generally true of the phylogenetic groups Spiralia and Deuterostomia
+ epidermis modified into an absorptive, syncytial tegument with protruding microvilli (Neodermata of Platyhelminthes)
* presence of external cilia that function in locomotion or fluid transport (goes along with living, exterior cells)
+ usually coincides with phylogenetic groups Spiralia or Deuterostomia, but
+ while the phylum Gastrotricha has cilia, the body is covered by a collagenous cuticle that covers and reduces the mobility of the cilia
this cuticle is not shed during growth
+ molecular evidence places Gastrotricha as the sister group of Ecdysozoa, but also related to Spiralia
* calcium carbonate spines, plates or shell, attached to underlying tissues, covering large areas, and growing by addition at edges as animal grows (most Mollusca)
* living cells permeating a non-living matrix (lorica) that is not shed during growth (Rotifera, Acanthocephala, Chaetognatha?)
* thin cuticle that grows with body (Annelida, etc.)

Digestive Tract

* absent (ancestral condition in Porifera and possibly Mesozoa, derived or secondary condition in some Platyhelminthes, Pogonophora, etc.)
* "blind"gut" (mouth but no anus), or
* complete (anus present)
* if complete, which subdivisions are recognized: + buccal cavity, pharynx, esophagus (foregut, or stomodeum),
+ stomach, crop, gizzard, proventriculus, small and large intestine (midgut),
+ rectum, colon, cloaca, which is a common chamber for digestive and excretory wastes and gamete passage (hind gut, or proctodeum).
+ presence of caeca, blind sacs that branch off the gut, usually from the stomach or intestine
+ number and form of accessory digestive organs: salivary and other oral glands, liver, pancreas, gall bladder,  rectal or anal glands


Whether or not body is metameric; if so:
* which body structures are repeated
* whether or not several types of repeated structures are coordinated into body units (tagmata)
* way in which metameres are formed in the embryo or juvenile + simultaneously, with number fixed, or
+ successively, just in front of pygidium, and numbers variable.  


If appendages are present:
* fleshy parapods with embedded chaetae + parapod number, structure, and position
+ parapods branched  (subdivided) or not
* lobopods (covered with cuticle, with terminal claws)
* jointed appendages supported by a chitinous exoskeleton
+ number and arrangement of separately muscled, walking leg branches and segments
+ number, shape, and enervation of head appendages
+ degree to which appendage sets are fused into single body units
* (jointed) appendages supported by a calcareous endoskeleton

Life History Stages

* whether or not marine members of the taxon have planktonic larvae + whether larvae swim by cilia, movable appendages, or flexible tail and fins
+ whether or not ciliated larvae have a circlet and an apical tuft of cilia (trochophore-like)
+ whether or not larvae have protonephridial excretory - osmoregulatory organs
* for primarily terrestrial, parasitic, or freshwater taxa, whether or not development is direct or indirect + if indirect, numbers and kinds of distinct, immature stages * whether they have, and if so what kinds of, diapause and cyptobiotic capabilities
* notable tendencies to retain juvenile or embryonic features in adults (paedogenesis, neoteny)

Reproductive Patterns

(These characters are often highly variable within taxa, and only generally useful as main trends across most species belonging to a taxon.)
* gonochoristic (diecious) or hermaphroditic (monecious ), and if hermaphroditic: + protandric or protogynous
+ cross-fertilization obligatory or optional
* whether fertilization is internal or external (often correlates with whether egg is shelled or not)
* whether embryos develop in utero or external to female; if in utero: + whether maternal nutrition is provided during development, or embryo relies entirely on yolk laid down at or before fertilization
+ if external, whether parent(s) invest(s) significant resources in nurture
* whether or not parthenogenetic; if parthenogenetic:
+ whether and under what circumstances males occur
+ whether males are haploid or diploid
+ whether or not sexual eggs or embryos are cryptobiotic
* position, number and form of genitalia and accessory sexual organs

Central Nervous System

* no cords or ganglia, or cephalic ganglia form a brain
* main, longitudinal nerve cords absent, multiple, or reduced
+ if reduced, how many and where (dorsal, lateral, ventral), are they
* locations of subsidiary ganglia, if any, in appendages, near sense organs, etc.
* patterns of nerves branching to appendages (if any)

Respiratory System

* if marine or freshwater:
+ gills present or absent
+ blood circulated through them or not
* if terrestrial:
+ cuticular respiration (simple diffusion from water or air to tissues through the skin or cuticle)
+ gills (protrusions from body surface with thin walls and large surface area)
- tracheal gills (water-tissue-tracheal air-tissue transfer sequence)
- blood gills (water-sinus blood fluid-tissue transfer sequence)
- gills supported by circulatory vessels (water-tissue-blood in capillary vessels-interstitial fluid-tissue transfer sequence)
+ a tracheal system (air delivered directly to tissues or cells)
- separate in each metamere
- weakly anastomosed, or
- with large, internal air spaces and longitudinal trunks (and spiracles often reduced to fewer metameres)
- with or without valves and filters near spiracles
- ending in blood sinuses which continue transport and distribution
+ internal lungs - associated with epidermis (pulmonary mantle cavity of terrestrial gastropods)
- associated with cuticle (book lungs ... thin-walled surfaces exposed to air within enclosed body cavities that open to the outside; many chelicerates)
- associated with digestive tract (esophageal; lungfishes and tetrapods) - relative size of lungs and complexity of lung walls (alveoli)
- circulatory supply (separated or shared heart ventricle)
- lung ventilation controlled by oral, chest wall or diaphragm muscles
- oscillating or once-through (parabronchii, birds) air flow

Circulatory System

* present or absent, if present:
+ open
- heart to force blood through open systems of vessels and sinuses
- heart lacking
+ closed
- contractile vessels drive circulation (Nemertea, Annelida, Cerphalochordata)
- heart to force blood through closed vessel circuits (Cephalopoda, Craniata)
^ multiple hearts (Cephalopoda)
^ single heart with multiple chambers (Craniata)

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Last modified on April 7, 2003