886 SUMMARY OF CURRENT RESEARCHES RELATING TO 



labrum of insects and the cheliceras of the former to the mandibles of 

 the latter. The circulatory as well as the muscular system approaches 

 that of Limulus, but the resemblance is to be explained, not by com- 

 munity of descent, but by the general similarity of shape. The 

 presence of a sac-like pericardium, the walls of which are continuous 

 with the pulmonary veins, recalls the dispositions met with in 

 Crustacea where the branchial veins are similarly prolongations of the 

 pericardium ; but this identity probably results from the disposition 

 of the respiratory organs, which in both groups are localized on the 

 lower surface of the abdomen. That this explanation is just, seems to 

 be proved by the fact that in the Opilionidas, where only trache£e are 

 present, the heart is not furnished with a pericardium. 



Spiders are furnished with " lungs " as well as tracheae, while in the 

 scorpion only the former kind of respiratory organ is present ; the 

 Opilionidfe as well as the Acarinas have only tracheae. Schim- 

 kewitsch disputes the opinion of Milne-Edwards that the lungs of 

 scorpions are comparable to modified branchiae of Limulus and suggests 

 rather that they have been formed by coalesced bundles of tracheae, 

 such as are to be found in many caterpillars ; he considers, however, 

 that the ancestors of both scorpions and mites breathed by means of 

 " lungs " and that the tracheae of the mites are a more modern develop- 

 ment. In no case do the respiratory organs of spiders show any 

 resemblance to those of Limulus. 



The generative organs show a great likeness to those of the 

 Pycnogonida. The genital glands in both groups are in the form of 

 a U, and situated above the intestine ; the position of the ovaries in 

 the legs in the Pycnogonida is quite secondary ; moreover, there is a 

 general correspondence in the appendages ; the rostrum of the Pycno- 

 gonida is entirely comparable to the upper and lower lip of spiders, 

 while the mandibles correspond to the chelicerfe. In support of this 

 homology it is stated that the chelicerae in some spiders, at any rate at 

 a certain stage of development, are composed of three joints as are 

 the mandibles of the Pycnogonida. The four pairs of legs are quite 

 similar in the two groups, and the palpi also. With regard to the 

 ovigerous limbs of the Pycnogonida, it seems at least probable that 

 they are the maxillae of the spider. In fact these two groups have 

 probably descended from a common ancestor ; the Pycnogonida being 

 in some respects arrested in development (articulate mandibles, free 

 thoracic segments) and in other respects modified (rudimentary 

 abdomen). 



Auditory and Olfactory Organs of Spiders.* — F. Dahl proposes 

 to classify spiders according to the character and disposition of the 

 auditory hairs on the limbs of these animals, as follows : 



1. Tibia with two series of auditory hairs, metatarsus with one 

 hair, and tarsus with a rudimentary pit or depression free from 

 hairs : e. g. Ejpeiridce, JJloboridoe,, Theridiidce, and Pholcidce. 2. 

 Tarsus with no rudimentary depression for auditory hairs, usually 



* Arch. f. Mikr. Auat., xxiv. (1SS4) pp. 1-10 (1 pi.). Also trausl. in Ann, 

 and Mag. Nat. Hist., xiv. (1884) pp. 329-37 (1 pi.). 



