Table 1.

Summary of the major groups of animals described in this review, and their invaginating and non-invaginating synaptic spines

Group	Invaginating Spines	Non-invaginating Spines
Porifera (sponges; no definitive neurons or synapses)	Possible spine-like invaginating processes1	None described
Ctenophora (comb jellies)	None described	Possible spine-like process on Colloblast2
Cnidaria (jellyfish, sea anemones, corals, hydroids)	Well-developed in photoreceptor synapses of cubozoan jellyfish3 also found in a hydroid4	None described
Flatworms (Platyhelminthes; Acoelomorpha)	Occasionally seen5	Well-developed6
Nematodes (roundworms)	None described	None described, but have postsynaptic spine-like muscle arms at neuromuscular synapses7
Chaetognatha (arrow worms)	None described	None described
Rotifera (rotifers)	Possible example8	None described
Phoronida (horseshoe worms)	None described	None described
Bryozoa (moss animals)	None described	None described
Annelida (leeches, earthworms, various marine worms)	None definitive	In leeches, spines on processes of large motor neurons9 Possible spines in other annelids10
Mollusca (gastropods like snails and sea hares, bivalves like clams, cephalopods like squid and octopi)	In photoreceptor terminals of squid and octopi11 In the octopus statocyst (balance and hearing)12 In synapses of the gill-withdrawal reflex in the sea hare13	In the stellate ganglion of squid, associated with giant axons for rapid escape jetting14 In synapses of the gill-withdrawal reflex in the sea hare15 Also in clams16
Arthropoda (horseshoe crabs, spiders, crustaceans like crabs and lobsters, insects)	In photoreceptor terminals of wolf spiders17 and lobsters18	Common in insect brain; good examples include Kenyon cell dendritic spines of the honeybee19 and those in a group of visual interneurons of Drosophila20 Other examples in spiders21 and crustaceans22
Onychophora (velvet worms)	None described	Possible spines23
Echinodermata (starfish, sea urchins, sea cucumbers)	One illustrated from a sea cucumber24	None described
Hemichordata (acorn worms, pterobranchs)	None described	None described
Invertebrate chordates (sea squirts or ascidians, amphioxus or lancelets)	At the base of coronal organ hair cells of a colonial ascidian25	In the larvae of the amphioxus (lancelet)26 and adult sea squirt27
Vertebrates (jawless fish including hagfish and lampreys, sharks and rays, bony fish, amphibians like frogs and salamanders, reptiles like lizards and turtles, birds, and mammals like rats, mice, rabbits and monkeys)	In photoreceptor terminals of all vertebrate groups; synaptic structure evolves from simple invaginating postsynaptic processes in some hagfish, to complexes of invaginating postsynaptic processes in other vertebrates28 At the base of hair cells of the auditory system, commonly in developing animals and sometimes in adults.29 Other examples include hippocampal and thalamic excrescences30, developing cerebellar Purkinje spines31, some filopodia-like spines of the red nucleus32, and some crest synapses33	Widespread on many kinds of neurons in all classes of vertebrates typically have an excitatory terminal on the head, and sometimes have one or two additional terminals on the side of the head or neck, with different neurotransmitters Include some unusual structures such as multiterminal spines34, gemmules35, and crest synapses36

Includes various postsynaptic spine-like protuberances as described in the text

References: Major references are included here. These and additional references are discussed in the appropriate sections of the text

¹Pavans de Ceccatty (1966)

²Franc 1978

³Yamasu and Yoshida (1976); Gray et al. (2009)

⁴Holtmann and Thurm (2001)

⁵Morita and Best (1966); Petralia et al. (2015)

⁶Keenan et al. (1981); Coss and Perkel (1985)

⁷White et al. (1986)

⁸Clément (1977)

⁹Purves and McMahan (1972)

¹⁰De Robertis and Bennett (1955)

¹¹Case et al. (1972); Cohen (1973); Dilly et al. (1963)

¹²Budelmann and Thies (1977)

¹³Bailey and Thompson (1979); Bailey et al. (1979)

¹⁴Hama (1962); Castejón and Villegas (1964)

¹⁵Bailey and Thompson (1979); Bailey et al. (1979)

¹⁶Kotsyuba and Kotsyuba (2002)

¹⁷Trujillo-Cenoz (1965); Trujillo-Cenoz and Melamed (1967)

¹⁸Hamori and Horridge (1966)

¹⁹Coss et al. (1980); Brandon and Coss (1982); Coss and Perkel (1985)

²⁰Scott et al. (2003); Leiss et al. (2009b)

²¹Strausfeld and Barth (1993); Strausfeld et al. (1993)

²²Brown and Wolff (2012); Wolff et al. (2012); Nässel (1977); Nässel and Waterman (1977)

²³Strausfeld et al. (2006)

²⁴Mashanov et al. (2006)

²⁵Burighel et al. (2003)

²⁶Lacalli (2002); Lacalli and Kelly (2003)

²⁷Dilly (1969)

²⁸Holmberg (1970, 1971); Holmberg and Ohman (1976); Haverkamp et al. (2000); Sterling and Matthews (2005)

²⁹Tanaka and Smith (1978); Whitehead and Morest (1985); Sobkowicz et al. (2003)

³⁰Hamlyn (1962); Spacek and Lieberman (1974); Martinez Guijarro et al. (1984); Chicurel and Harris (1992); Petralia and Wenthold (1992); Petralia et al. (2015); Wiera and Mozrzymas (2015)

³¹Meek and Nieuwenhuys (1991); Castejón and Apkarian (1993); Altman and Bayer (1997)

³²Wilson et al. (1987)

³³Akert et al. (1967a)

³⁴Gulyas et al. (1992); Acsády et al. (1998)

³⁵Rall et al. (1966); Price and Powell (1970a, b)

³⁶Akert et al. (1967a, b); Yamashita et al. (1997)