THE CRIBELLATE SPIDERS 139 



phylogeny has to do with the origin of the cribellate spiders and 

 their relationship to the ecribellate families. By some they are held 

 to be a homogeneous group derived from a single line of ancestral 

 spiders that put their fading anterior median spinnerets to a new 

 and original use by inventing the calamistrum. On the other hand, 

 the cribellate spiders can be regarded as a remnant held over from 

 a time when all spiders were cribellate, and the modern forms can 

 then conceivably originate from one or several distinct lines. The 

 presence among these spiders of some in which the second pair of 

 book lungs is still persistent suggests a very early origin for the 

 group, and also strongly indicates that all ancestral spiders were 

 provided with cribellum and calamistrum. If we subscribe to this 

 belief, then it can be put that the ecribellate spiders have lost the 

 spinning organs, rather than that the cribellates have gained them. 

 In this book we consider the cribellate spiders in a single chapter, 

 even though among them are certain discordant elements that sug- 

 gest a multiple origin. 



THE FOUR-LUNGED TRUE SPIDERS 



One of the oldest American spiders is Hypochilus thorelli, a 

 strange relic of the past, whose forebears were probably aerial con- 

 temporaries of the Paleozoic ground tarantulas. The only known 

 living relatives of Hypochilus are two species of a related genus, 

 Ectatosticta, found in China and Tasmania. Although we regard 

 spiders of the family Hypochilidae as being true spiders, they share 

 many of the features of the tarantulas, the most notable being pos- 

 session of the posterior pair of book lungs, which in all other true 

 spiders have been transformed into tracheae. These lungs are situated 

 beneath and about at the middle of the abdomen, and their spiracles 

 open at the sides of a prominent furrow. The chelicerae are pro- 

 vided with venom glands entirely contained within the basal seg- 

 ment, and the heart has four pairs of ostia as in the tarantulas. 

 Perhaps the most distinctive badge of the true spider is the articula- 

 tion of the chelicerae. In Hypochilus we find the chelicerae largely 

 intermediate in type between those of the true spiders and those of 

 the tarantulas; since the claws do not point toward each other, they 

 are in many respects nearer those of the latter. The cribellum of 

 this spider is a rounded plate lacking the median dividing ridge but 

 pinched before and behind to indicate its original dual character. 



