LIMULUS AN ARACHNID. 623 
In Crustacea generally they are immobile and of very various 
shapes; but in Cirrhipedia, and possibly some others, they 
are filamentous, with a motile tail. In chilopod Myriapods 
they have a vibratile tail. In Chilognaths they are motion- 
less. In hexapod Insects they have a vibratile tail. In some 
Arachnida (e.g. Spiders) they are devoid of such a process. 
We owe to Kolliker the observation that in the Scorpio 
europeus the spermatozoa are filamentous in form, with a 
vibratile tail. Accordingly, it is compatible with Arach- 
nidan affinities for the spermatozoa to be either motile or 
immobile ; at the same time, as an element in the cumulative 
evidence of affinity between the King Crab and Scorpion, 
which it has been my object in this essay to bring together, 
the presence of vibratile spermatozoa in Limulus is a fact 
of value. The spermatozoa of Limulus are, as I observed 
four years ago (4), provided with a long vibratile tail; they 
agree, therefore, with those of the Scorpion. 
C. Tor Euryprerina AS A ConneEcTING LINK 
BETWEEN LIMULUS AND SCORPIO. 
The intimate affinity of the extinct Eurypterina with 
Limulus is no longer doubted. The researches of Hall, 
Huxley, and Woodward, have thoroughly established the 
fact that Pterygotus, Kurypterus, Slimonia, and Stylonurus, 
are to be regarded as Limuli, in which one pair of leg-like 
organs (probably the most anterior) has been suppressed, 
and in which the telsonic region, instead of exhibiting but an 
imperfect development of segments posterior to the twelfth, 
and that only in the embryo, gives rise to a series of segments 
forming a large tail-like region of the body. The result of 
this development of segments between the anus and the last 
appendage-bearing segment (the twelfth of Limulus) is that 
the so-called “ macrourous ’’ form of body is produced, and 
consequently a general similarity in appearance is observed 
between the Eurypterina and Scorpio. 
The two woodcuts (figs. 19 and 20) sufficiently exhibit 
this general resemblance. In other respects, allowing for 
the suppression of an anterior pair of appendages in the 
Eurypterina, we find obvious agreements with Limulus. 
The actual fifth pair of limbs—theoretically the sixth— 
present constantly in all the genera that enlarged form and 
specialisation of their terminal joints which are noticed in 
the corresponding limbs of the King Crab. The coxe of 
these and of the three pairs of limbs in front are brought up 
to the mouth, and denticulated so as to serve as jaw-organs, 
