Memoirs: The Phylogeny of the Tracheæ in Araneæ

The theoretical suggestions in the preceding paragraphs may be summed up as follows: In the first place I suppose the saccules of the second pair of lung-books to have been converted into tracheal tubules in the common ancestor of the Dysderidæ, Oonopidæ, and Caponiidæ. The resultant tracheæ then increased in size, and, as the number of the leaves of the anterior lungbooks decreased in inverse ratio, the former became the priucipal organs of respiration. The second pair of spiracles retained their position, or may even have moved slightly forwards, and the conversion oE the entapophyses into trachea? could not take place here, and would, moreover, be quite unnecessary. In the Caponiidaa the anterior pair of lungbooks were converted into tracheae in a similar manner, but at a, later period, and independently of the conversion of the posterior pair; but as the latter already provided almost the entire body with tracheæ, the anterior pair did not further increase in size. In the second place, iu the progenitor (or progenitors) of the remaining tracheate spiders, the posterior lung-books became reduced in size and effectiveness by the disappearance oE their saccules, accompanied by an increase in the number of the leaves of the anterior lung-books. Further, the posterior spiracles became approximated and united to a single spiracle, and moved towards the hinder end of the body, thereby causing the entapophyses of the tracheal segment to elongate. In this condition the Filistatidas, Sicariidse, and Palpimanidas have remained, with slight modifications, such as the division of the tracheal antechambers into branches in some forms. In the great majority of the families, however, the elongated entapophyses became transformed into a pair of medial tracheal trunks, thus producing a tracheal system consisting of four simple unbranched trunks, which is still found in some genera at least, in nearly all the families. A new factor having been introduced, viz. the presence of the respiratory entapophyses lying in the large ventral sinus containing venous blood requiring aeration, we accordingly find the second respiratory segment again taking a prominent part in the respiration in many forms, owing to the increase in size and the branching of the medial trunks, accompanied ultimately by a corresponding reduction in the size of the anterior lung-books, e. g. in the Attidæ. This method of origin of the tracheæ is independent of that of the Dysderidse and its allies, and the tracheal tubules, when present, would here not be derived from saccules, but be new formations.