308 II err J, Wagner on the 



tr.ica as \vell as the Araelinoidea were completely differen- 

 tiated and the postenibryonic nietamor])hosis no longer 

 pursued its primitive course, as is proved by the embryology 

 of the Scorpion. The author takes cognizance of external 

 characters alone, without touching upon the internal organi- 

 zation of the Arachnida or the Crustacea, or upon embryo- 

 logical tacts. Greater interest for us is to be found in the 

 circumstance that Oudemans professes to support Haller's 

 attempt to separate the Acarina into an independent group — 

 Acaroidea, Hall. Besides Haller's reasons, namely the 

 arrangement of the mouth-parts, and in accordance therewith 

 the different number of the appendage-bearing segments, the 

 boundary between the cephalothorax and the abdomen {i. e. 

 according to Hallcr between the second and third pairs of 

 legs), and a hexapod larval stage ("Ca?'w-stage"), Oudemans 

 also states his own — the absence of the heart in the Acarina, 

 the absence of the endosternite, and especially the position of 

 the movable finger in the chela?. The insufficiency of 

 Haller's arguments has been demonstrated in my paper on 

 the development of Ixodes (No. 66) ; as regards the argu- 

 ments of Oudemans himself, however, only the last of them 

 deserves attention. Yet naturally this one character without 

 others is quite inadequate to separate the Acariua as a special 

 group of the Arthropods. The first two arguments are, as is 

 well known, according to the latest observations, false. If 

 the author's last argument {loc. cit. pp. 45-46), namely that 

 in all Crustacea, Pantopoda, and Acarina the movable joint 

 or the finger of the cheJse is situated on the outer side, but in 

 Limulus and all Arachnida (except the Acarina) is directed 

 inwards, should be confirmed by further observations, two 

 explanations are here possible. On the one hand, we might 

 suppose, since certain forms of the Gigantostraca had no 

 cheJffi, that the alteration in the primitive relation of the two 

 last joints of the ftet to one another proceeded independently 

 iu the difierent groups of the Arachnoidea (and of the 

 Crustacea) * ; on the other, the chelicerae perhaps change 

 their original position in the movement towards the front, as 

 I have shown to be the case in the pedipalpi (No. 66, p. 67 ; 

 the common base of the latter moves during development 

 from the longitudinal into the transverse direction with 

 reference to the primitive streak). The first explanation 

 appears to me to be the more probable ; in any case, the 

 circumstance that pincers are an ordinary weapon of certain 



* Certain Arachnids have, as is well known, no chelae at all ; their loss 

 by these forms (Araueiua, Fhrymis, and certain Acariua) is only 

 secondary. 



