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.
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.
+ protostome animals in the phylogenetic group Spiralia usually have spiral cleavagebut 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,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).
+ lined (eucoelomate) and formed by splitting of mesodermal cell masses (schizocoelous), or
+ lined (eucoelomate) and formed by outpocketing of the endodermal archenteron (enterocoelous)
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.
+ usually coincides with phylogenetic groups Spiralia or Deuterostomia, but* calcium carbonate spines, plates or shell, attached to underlying tissues, covering large areas, and growing by addition at edges as animal grows (most Mollusca)
+ 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
+ number and arrangement of separately muscled, walking leg branches and segments* (jointed) appendages supported by a calcareous endoskeleton
+ number, shape, and enervation of head appendages
+ degree to which appendage sets are fused into single body units
+ whether and under what circumstances males occur* position, number and form of genitalia and accessory sexual organs
+ whether males are haploid or diploid
+ whether or not sexual eggs or embryos are cryptobiotic
+ if reduced, how many and where (dorsal, lateral, ventral), are they* locations of subsidiary ganglia, if any, in appendages, near sense organs, etc.
+ gills present or absent* if terrestrial:
+ blood circulated through them or not
+ 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)+ a tracheal system (air delivered directly to tissues or cells)
- 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)- separate in each metamere+ internal lungs
- 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
- 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
+ open- heart to force blood through open systems of vessels and sinuses+ closed
- heart lacking- 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)
Last modified on April 7, 2003