Savory - Arachnida 1977

174 Ill. PROLES ARACHNES
22
The Order Haptopoda
''
'' L-'
,,
'"
Frc. 61. Cryptomartus. After Petrunkevitch.
other Arachnida, is the unusual way in which the first or basal segments
of the chelicerae are placed between the coxae of the pedipalpi, so that
all four appendages originate at the same level. At the same time, and
as a result, the mouth is moved to a position immediately in front
of the sternum, where it therefore lies between the coxae of the first
legs. There is little doubt that the various configurations of the mouth
parts as they occur in the orders of Arachnida, as, indeed, elsewhere
among the Arthropoda, often provide evidence of evolutionary relations.
The sub-class Stethostomata is a particularly interesting one for this
reason and bears all the appearance of an unsuccessful combination of
genes. One cannot but regret the fact that today we have no chance
of seeing one of the Stethostomata consuming its food. A special kind of
food may have been necessary, imposing an unwelcome limitation on
the animal's everyday life. If environmental conditions changed these
difficulties may have increased, and may have been among the causes of
extinction.
'' I I
LJ
[Haptopoda Pocock, 1911; Haptopodida Petrunkevitch, 1955]
Carboniferous Arachnida in which the prosoma is covered by an undivided
carapace, bearing two tryes close to the fore-edge. The sternum is long and
narrow and is of three sternites. The opisthosoma is of 11 somites, the
first tergite being very short and wholly or partly hidden by the carapace.
The .first sternite is triangular, between the coxae of the fourth legs; the
second is very large, convex behind, with two genital orifices. Sternites 3
to 10 are longitudinally divided by a median line. The chelicerae are of
three segments, chelate. The pedipalpi are of six segments and have no gnathobases.
The legs are of seven segments, with their tarsi sub-segmented, and
are of relative lengths 1.4.3.2.
This order, founded by Pocock in 1911, contains but one family, the
Plesiosironidae, with one genus, Plesiosiro, and a single species, P.
Frc. 62. Plesiosiro madeleyi. After Petrunkevitch.