278 ON THE SPINNIIir a- GLANDS IN PHETNUS. 



a waist or diaptragm, accounts for the secondary degeneration of 

 the limbs in this latter region. 



From the operculum of ThelypTionus both the projecting limbs 

 have now disappeared, as is also the case in many Phrynidse. 

 Their disappearance is, however, marked in the latter by the pair 

 of rounded membranous eminences bearing the claw-like rods 

 described and iigured by Pocock, and perhaps also in the former 

 by certain chitinous ridges visible on raising the operculum. 



The fact that the " penis " is clutching what looks like the 

 remains of a cocoon (fig. 1), and, from what we have seen, might 

 quite as well be an ovipositor as a penis, inclines me to think 

 that the occasional presence of these limbs may be reversionary, 

 and not in any way indicative of sex. It is possible that we have 

 here a case of dimorphism. Whereas a majority of the Phrynidse, 

 and, indeed, of Arachnida, have lost the distal portion of the 

 genital limbs, they may occasionally reappear in the Phrynidse, 

 in which group perhaps, to judge from the character of the 

 operculum, they persisted longer than in those Arachnids in 

 which the opercula are now reduced to mere scales. 



EXPLANATION OF PLATE VIII. 



Fig. 1. Three anterior abdominal segments of Tarantula tessellata, Poc, ventral 

 surface, showing the so-called " penis " tightly clutching a small 

 fragment of a cocoon. 



2. The ventral (morphologically anterior) view of the " penis," after 



removing the opercular fold, showing the mass of the (left) gland 

 which secretes the material for the cocoon. 



3. One tip of the same more highly magnified, showing the delicate tips of 



the organ. The gland opens somewhere among the folds at the inner 

 base of these delicate tips. 



4. Dorsal (morphologically posterior) view of the limbs forming the 



" penis ; " deep down in the channel between them anteriorly is the 

 genital aperture. 



5. One tip of the same, more magnified. 



6. Diagrammatic longitudinal section to illustrate the position of the 



*' penis" when not used, and of the secreting -gland. 



7. Diagram to show the relation of the limbs of the genital segment to 



those of the next following segments, to illustrate the probable origin 

 of the large genital operculum of the Pedipalpi {cf. fig. 1). 



