"LIMULUS AN ARACHNID, 615 
Limulus and Scorpio. The exceeding shortness of the 
proctodzeum of Limulus is only a part of that general reduc- 
tion of its hinder segments which is paralleled in many 
other groups of Arthropoda, But in Scorpio there are given 
off from the anterior border of the proctodzeum two delicate 
tubes. According to Dufour, in Sc. occttanus there are four 
of these tubes, of which one pair is branched. Newport, 
however, figures only one pair in Buthus. These delicate 
tubes are the Malpighian glands, found alike in Myriapoda, 
Hexapoda,and Arachnida, but never in Crustacea. They have 
been shown in Spiders by Mr. Balfour (20) to develop from 
the proctodzum, or anal invagination of the epiblast; they 
have a renal function, and possibly represent morphologi- 
cally ‘ nephridia,’ such as those of Gephyrea and Rotifera. 
Their absence from Limulus is a difficulty in the way of 
associating Limulus and Scorpio, but itis also a difficulty in 
the way of associating the Crustacea with the other Arthro- 
poda. Leydig has pointed out, in the proctodeum of Cope- 
poda, structural evidence of the existence of a region which 
may functionally represent the Malpighian tubes of the 
tracheate Arthropoda, and careful histological study may give 
similar evidence with regard to Limulus. As to the develop- 
ment of actual cecal tubes in this region, two views are admis- 
sible: either the common ancestor of the Arthropoda possessed 
these tubes and they have been lost by Crustacea and by 
Limulus (and some others) among the Arachnida, but 
retained by the various tracheate classes, or the common 
ancestor possessed only the functional ‘‘renal region” of 
the proctodeum, which has remained undifferentiated in 
form in Crustacea and in Limulus, but has taken on the form 
of czecal tubes in the air-breathing forms, perhaps indepen- 
dently, in the course of the evolution of different groups. If 
we are to hold that Malpighian tubes can only once have 
originated, and that all forms possessing them have a common 
ancestor, we must suppose either that Limulus has lost them 
or that all Tracheata are descended from the Arachnida. 
Amongst these possibilities we have no decisive indications. 
The whole question of the genealogy of the various classes of 
Arthropoda is involved in the issue. 
§ d. VASCULAR SYSTEM.—The close agreement between 
the vascular system of Limulus and Scorpio has been ably 
insisted upon by M. Alphonse Milne-Edwards, who, eight 
years since, gave the results in his beautiful memoir, already 
cited, of a series of injections carried out upon perfectly fresh 
specimens of Limulus. It is not possible to say, in the 
