1909] Evolution in Palpus of spiders 61 



primitive type, and moreover it generally coincides with the 

 long axis of the bulb, whenever this is in evidence. This line or 

 axis is a valuable basis for orientation in the manifold changes in 

 form undergone by the bulb in the higher families, and I will, 

 for convenience, term it the principal axis of the bulb. This axis, 

 as the figure shows, generally forms an angle of from 50 to 90 

 degrees with the long axis of the tarsal joint. Four characters are 

 to be noted as marking this type of palpus : (a) the more or less 

 cylindrical form of the tarsal joint; (b) the position of the bulb, 

 frequently terminal and rarely, if ever, near the base of the tarsus; 

 (c) the narrowness of the zone of articular membrane joining cym- 

 bium and bulb, scarcely wider than that between the joints of 

 other appendages; (d) the integument of the bulb itself, which is 

 a continuous chitinized sheet, not broken up into segments or 

 plates. This type of bulb I propose to designate by the term 

 holochittJious. 



Turning to the palpal organ of a representative of the higher 

 spider families in the Dipneumones (figs. 2-7) it is seen that the 

 primitive type just described has undergone great alteration, 

 especially in regard to one or more of the four characters des- 

 cribed. These are, broadly speaking, the following: (a) change 

 in the form of the tarsal segment or cymbium; (c) this has be- 

 come compressed in a plane at right angles to that in which the 

 bulb lies, so as to present an elliptical or oval outline in face view, 

 and is moreover hollowed out beneath the base of the bulb to 

 form the alveolus (a) ; (b) The position of the bulb is uniformly 

 near the base of the cymbium, never, in the higher groups, near its 

 apex; (c) the articular membrane joining bulb and cymbium 

 has increased greatly in extent, and has become an elastic sac 

 (figs. 6 and 7) capable of being greatly distended l)y blood pres- 

 sure, thus erecting the bulb, thrusting it out some distance from 

 the alveolus and away from the cymbium. It is accordingly 

 termed the haematodocha ; (d) The bulb itself has suffered alter- 

 ation in that it is now generally split transversely into two or 

 more segments, united by membranous integument. It is also 

 frequently greatly shortened in its principal axis. 



Each one of the above mentioned alterations of the primitive 

 type seem quite evidently directed toward one end, namely, 

 protection of the bulb, the essential part of the copulatory organ, 

 from mechanical injury. Taking the above four named altera- 

 tions in order: (a) Change in the shape of the tarsal joint, from 



