562 The Inter-Relationships of Arthropods. [June, 
organs, have only recently been known. Packard first found 
them in Limulus, and later Lankester made an exhaustive histo- 
logical study of them in comparison with similar glands occur- 
ring in the scorpions. Neither of these authors were able to find 
any external duct. More recently Bertkau has found that in the 
young spider they open externally at the base of the fifth pair of 
appendages (third pair of legs), but this duct is lost in the adult. 
The studies of Mr. Michael on the coxal glands of mites are, so 
far as they go, confirmative. In Limulus I found that they arose 
as coiled mesoblastic tubes, closely comparable to the segmental 
organs of worms, and emptied in the young at exactly the same 
point as in spiders. 
It seems to me that these various points show that the Acerata 
are more closely allied to the Crustacea than to the hexapods and 
myriapods with which they are usually classed. Lankester has 
shown from a study of Apus that all of the crustacean appen- 
dages are primitively post-oral, a fact which is confirmed by the 
study of the development of various forms. With this point set- 
tled it would appear that we have a perfect right to compare, 
within certain limits, the segments and their appendages of spi- 
ders and crustaceans. Tabularly arranged, the result would be 
somewhat as follows : 
CRUSTACEA. ACERATA. 
1, Antennula, Chelicera, 
II. Antenna Pedipalpus 
tt. Mandible, Leg 1. 
Iv. Maxilla 1. Leg It. 
v. Maxilla 2. Leg 111. 
vi. Maxilliped 1, Leg Iv. 
That this serial comparison is legitimate is shown by several 
reasons besides that given above. In the Acerata the series em- 
braces all of the cephalothoracic appendages. In the Crustacea 
it stops exactly at the line of division between the so-called head 
and thorax of the tetradecapods as well as in the embryos of 
many other forms. A further point is interesting. In many of 
the lower Crustacea a pair of so-called shell glands occur which 
are regarded by Claus, Grobben and others as comparable to the 
segmental organs of worms. The outlets of these organs occur 
-~ at the bases of the second pair of maxillz, a position which the ~ 
~ above table will show is exactly comparable to that of the outlet 
__ of the coxal glands of Limulus and the spiders. In many Crus- 
