LIMULUS AN ARACHNID, 621 
one. The tubular genital gland is not disposed as a simple 
central body with two ducts, nor as right and left lobes united 
by a central isthmus, nor as a single or double bunch of simple 
or arborescent ceca, but it is distinctly retiform. There 
are two genital ducts, which pass from the two genital pores 
right and left, and are continued into a widely diffused 
meshwork. ‘The meshwork may be regarded as a continua- 
tion of the two genital ducts which give rise to branches, 
which anastomose and also join their fellows of the opposite 
side ; it hasa tubular structure, and its walls present follicles 
in which the generative cells are produced. In Scorpio the 
ovarian follicles are less numerous and more highly developed 
individually than in Limulus, and also in the former animal 
the meshwork formed by the gland is more symmetrical and 
its meshes larger than in the latter, but the reticular arrange- 
ment of the genital gland is the same in both. 
The main differences in the genital glands of Limulus, as 
compared with those of Scorpion, are related to two modifying 
causes : firstly, the greater relative size of the cephalothorax 
in Limulus; and, secondly, the terrestrial mode of life of 
Scorpion which replaces the aquatic mode of life of Limulus. 
Owing to the first of these causes we find that, whereas 
in Limulus the retiform generative gland extends both in 
front of and behind the genital pore, that is to say, into the 
cephalothorax (segments 1 to 6) and into the abdominal 
segments (segments 7 to 13), in Scorpio we find its mesh- 
works spread entirely inthe region posterior to the genital pore, 
that is, in the wide and thick abdominal segments (7 to 18). 
The second cause has brought about a very important 
difference in the secondary arrangements of the generative 
system. Limulus does not copulate, but the male discharges 
the spermatozoa into the water on to the surface of the eggs 
which have just been laid by the female. Such a method of 
fertilisation is impossible in any animal of strictly terrestrial 
habits. Copulation is a necessity in such animals. It is 
only those terrestrial animals which pass into the water 
during the breeding season which can dispense with intro- 
mission. Accordingly we find the efferent ducts, both in 
male and female Scorpions, modified to subserve copulation. 
The ejaculatory apparatus in the male is complicated; the 
distal portion of each of the efferent ducts is modified so as 
to form an intrommittent organ, and accessory glands are 
developed from its sides. The two oviducts in the female 
are enlarged to form vagine. There are thus two penes and 
two vaginez in the male and female Scorpion respectively. 
In copulation the female appears to lie upon her back and, 
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