PSEUDOSCORPIONES. 31 



(Fig. 17 B). On the abdomen, which is bent ventrally, four pairs 

 of limb-rudiments appear (Fig. 17 B), which, however, soon com- 

 pletely degenerate. The Pseudoscorpiones agree in this respect with 

 other Arachnida. The most anterior pair of limbs is still wanting, 

 but a paired thickening is found dorsally, above the base of the 

 proboscis ; this has apparently arisen from an invagination, and is the 

 rudiment of the supra-oesophageal ganglion (Fig. 16 B, g). This 

 recalls the cephalic pits of the Scorpiones and Araneae (pp. 12, 53). 



The larva continues to approach the adult in form, segmentation 

 appearing both in the limbs and in the abdomen, but the cephalo- 

 thorax remains unsegmented. The chelicerae have, in the meantime, 

 appeared in front of the pedipalps. The true upper lip arises 

 between them, some way from and altogether independent of the 

 larval proboscis (Fig. 17 C). The proboscis degenerates, the last 

 vestige of it being lost when the larva moults, at the stage depicted 

 in Fig. 17 C. It is then still found attached by a delicate thread 

 to a point behind the future mouth, until it is cast off with the 

 larval integument (Barrois). A large mass of yolk can still be 

 seen within the body, enclosed in the enteron, which opens exter- 

 nally through the proctodaeum at the posterior end of the body 

 (Fig. 17 C, an.i). The oesophagus is probably also formed by an 

 ectodermal invagination (Metschnikoff). 



General Considerations. The ontogeny of the Pseudoscorpiones 

 is remarkable on account of the embryo leaving the egg-membrane 

 with a much simpler structure and at a much earlier stage than in 

 other Arachnida. Further, the larvae, in their half parasitic life 

 on the body of the mother, have developed a provisional sucking 

 organ which at first lies in front of the first pair of limbs, but 

 shifts back later, in consecpiience of processes of growth, on to the 

 ventral surface (Figs. 16 and 17); this organ, however, cannot be 

 compared to a pair of limbs. No homologue has so far been 

 discovered among the Arachnida for this proboscis, which must 

 therefore be regarded as an organ acquired by the Pseudoscorpiones 

 through their peculiar method of development. 



The difference between the ontogeny of the Pseudoscorpiones and that of the 

 Scorpiones, to which they are perhaps most nearly related, is very striking. 

 The cleavage, the formation of the blastoderm, and the first rudiment of the 

 embryo in the two forms can hardly be compared. They also differ in impor- 

 tant points of their organisation. The absence of the taildike abdomen, the 

 disappearance of the abdominal ganglia (Ckoneberg), the position of the genital 

 apertures (in the second abdominal segment), and, not least, their tracheal 

 respiration, remove the Chernetidae from the true Scorpiones so far that the 



