1909.] NATURAL SCIENCES OF PHILADELPHIA. 301 



early stages of the Aviculariid Evagrus — this being to my knowledge 

 the first account of the embryology of any Theraphosid. The earliest 

 stage possessing extremities found is shown in fig. 28, Plate XIV, a 

 lateral view. Next to the head lobe is found the cheliceron (Chel.), 

 then the pedipalp (Pcd.), then the four pairs of thoracal legs (L. 1- 

 L. 4). The abdomen is remarkable in possessing only two segments 

 (1, 2), whereas in other spiders at a corresponding stage the abdomen 

 would be richly segmented, and in having a huge caudal lobe (C. L.) 

 projecting forward. The next stage available is that of fig. 29, a 

 ventral view of the abdomen: the two pairs of lung books (Pul.) have 

 formed, the nerve ganglia (N. C.) are clearly marked, and there are 

 indications of three pairs of spinnerets. Of the latter the anterior 

 (A. Sp.) are only faintly marked thickenings, and evidently belong 

 to the fourth abdominal segment; the median spinnerets (ill. Sp.) 

 are rounded projections just median to and in the same plane with 

 the largest posterior spinnerets (P. Sp.), therefore outgrowths of the 

 fifth segment. Three successive later stages, shown in figs. 30-32 

 respectively, show that the anterior spinnerets have disappeared and 

 that it is those of the fifth segment, the median and the posterior, 

 that persist into the adult. Thus the fourth segment, as shown clearly 

 in figs. 30 and 31, produces no respiratory organs, but originates a 

 pair of rudimentary spinnerets that subsequently disappear. 



The two pairs of spinnerets of Evagrus, and of probably also other 

 Aviculariids with that number, correspond to the median and posterior 

 spinnerets of those araneads with three pairs; but the development 

 shows that Evagrus retains in the embryo a slight trace of the anterior 

 pair. This homology has been already suggested by Simon, on the 

 basis of the relative position of the parts in the adult. 



In the next place we may consider the colulus, an organ that has 

 received only the scantiest attention. It may be best studied in 

 Sicariids where it attains its greatest size, and the following account 

 applies to the genus Loxosceles. 



In an adult individual the form and position of the colulus is shown 

 on ventral view in fig. 26, PI. XIII. It is an unpaired conical projec- 

 tion (Col.) between the anterior spinnerets (A. Sp.), its base conflu- 

 ent with their bases and at some distance behind the tracheal stigma 

 (T. St.). The transverse line in the figure which crosses it from left 

 to right represents the border of a skin-fold just dorsal to it and to the 

 spinnerets, and this fold is deeply indented in the median line. Its 

 free apex is directed ventro-caudad; its surface bears simple hairs 

 like those on the ventral surface of the abdomen, and the small circles 



