On 7/23/25 08:30, erik simpson wrote:

The unusual structure of arachnid brains has roots in the lower Cambrian.

https://www.cell.com/current-biology/fulltext/S0960-9822(25)00822-X?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS096098222500822X%3Fshowall%3Dtrue

Summary
Fossils from the lower Cambrian provide crucial insights into the diversification of arthropod lineages: Mandibulata, represented by centipedes, insects, and crustaceans; Chelicerata, represented by sea spiders, horseshoe crabs, and arachnids—the last including spiders, scorpions, and ticks.1 Two mid-Cambrian genera claimed as stem
chelicerates are Mollisonia and Sanctacaris, defined by a carapaced
prosoma equipped with clustered limbs, followed by a segmented trunk opisthosoma equipped with appendages for swimming and respiration.2,3,4 Until now, the phyletic status of Mollisoniidae and Sanctacarididae has
been that of a basal chelicerate,2 stemward of Leanchoiliidae, whose neuromorphology resembles that of extant Merostomata (horseshoe crabs).5 Here, we identify preserved traces of neuronal tissues in Mollisonia symmetrica that crucially depart from a merostome organization. Instead,
a radiating organization of metameric neuropils occupying most of its prosoma is situated behind a pair of oval unsegmented neuropils that are directly connected to paired chelicerae extending from the front of the prosoma. This connection identifies this neuropil pair as the
deutocerebrum and signals a complete reversal of the order of the three genetically distinct domains that define euarthropod brains.6 In
Mollisonia, the deutocerebrum is the most rostral cerebral domain. The proso- and protocerebral domains are folded backward such that tracts
from the principal eyes extend caudally to reach their prosocerebral destination, itself having the unique disposition to interact directly
with appendicular neuromeres. Phylogenetic analyses employing
predominantly neural traits reveal Mollisonia symmetrica as an upper
stem arachnid belonging to a lineage from which may have evolved the planet’s most successful arthropodan predators.

I guess the nervous system of animals takes in sensory information
from the environment and then use specialized cells to coordinate
movement of a large multicellular organism often using specialized
tissues called 'muscles' to facilitate movement.

Nonetheless the external environment in an organism that moves can
change pretty quickly, thus producing the need for sensory organs
that can help to make movements beneficial to the organism.

A long time ago when reading about the brains of insects I noticed
that it like vertebrates generally had a similar sensory - dorsal,
motor - ventral nerve pattern, but also noticed how large was the
part of the brain that coordinated information from the compound
eyes. Arachnids do tend to have similar compound eye processing
systems in the brain to that of insects? Optical processing in
arachnids is vastly different from that of insects?

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On 7/23/25 19:45, erik simpson wrote:

On 7/23/25 12:24 PM, x wrote:

On 7/23/25 08:30, erik simpson wrote:

The unusual structure of arachnid brains has roots in the lower

Cambrian.

;
;

https://www.cell.com/current-biology/fulltext/S0960-9822(25)00822-X?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS096098222500822X%3Fshowall%3Dtrue

;
Summary
Fossils from the lower Cambrian provide crucial insights into the
diversification of arthropod lineages: Mandibulata, represented by
centipedes, insects, and crustaceans; Chelicerata, represented by sea
spiders, horseshoe crabs, and arachnids—the last including spiders,
scorpions, and ticks.1 Two mid-Cambrian genera claimed as stem
chelicerates are Mollisonia and Sanctacaris, defined by a carapaced
prosoma equipped with clustered limbs, followed by a segmented trunk
opisthosoma equipped with appendages for swimming and

respiration.2,3,4

Until now, the phyletic status of Mollisoniidae and Sanctacarididae

has

been that of a basal chelicerate,2 stemward of Leanchoiliidae, whose
neuromorphology resembles that of extant Merostomata (horseshoe

crabs).5

Here, we identify preserved traces of neuronal tissues in Mollisonia
symmetrica that crucially depart from a merostome organization.

Instead,

a radiating organization of metameric neuropils occupying most of its
prosoma is situated behind a pair of oval unsegmented neuropils

that are

directly connected to paired chelicerae extending from the front of

the

prosoma. This connection identifies this neuropil pair as the
deutocerebrum and signals a complete reversal of the order of the

three

genetically distinct domains that define euarthropod brains.6 In
Mollisonia, the deutocerebrum is the most rostral cerebral domain. The >>  > proso- and protocerebral domains are folded backward such that tracts
from the principal eyes extend caudally to reach their prosocerebral
destination, itself having the unique disposition to interact directly >>  > with appendicular neuromeres. Phylogenetic analyses employing
predominantly neural traits reveal Mollisonia symmetrica as an upper
stem arachnid belonging to a lineage from which may have evolved the
planet’s most successful arthropodan predators.

I guess the nervous system of animals takes in sensory information
from the environment and then use specialized cells to coordinate
movement of a large multicellular organism often using specialized
tissues called 'muscles' to facilitate movement.

Nonetheless the external environment in an organism that moves can
change pretty quickly, thus producing the need for sensory organs
that can help to make movements beneficial to the organism.

A long time ago when reading about the brains of insects I noticed
that it like vertebrates generally had a similar sensory - dorsal,
motor - ventral nerve pattern, but also noticed how large was the
part of the brain that coordinated information from the compound
eyes.  Arachnids do tend to have similar compound eye processing
systems in the brain to that of insects?  Optical processing in
arachnids is vastly different from that of insects?

Arachnid eyes are different from the compound, multi-faceted eyes of insects.  Arachnid eyes have simple lensed eyes.  Depending on species there may be several (up to eight) or none.

I am thinking I should look up the retinas of arachnids some.

You know the retinas of vertebrates, mammals, and humans do
a lot of image processing that is not in the brain itself.

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