LIFE-FORMS OF THE PAST AND PRESENT. 
395 
Crustacea), are divided into two sub-orders, the Eurypterida and 
the Xiphosura. These may not inaptly be compared with the 
higher Decapoda, which are divided into Brachyura (short- 
tailed) and Macrura (large or long-tailed) ; for in the Euryp- 
terida the segments of the body are distinct and well developed 
(See Plate XC., figs. 1, 2, 3, 4), whilst in the Xiphosura or 
“ King-crabs,” they are coalesced and cephalised (Plate XC., 
figs. 6, 7, 8, 9). In both, however, the details of their organi- 
sation are arranged upon a common pattern. 
In the Crab and Lobster the compound eyes are placed on 
movable eye-stalks ; the antennse, the mouth-organs and the 
organs of prehension, locomotion, and respiration, are all con- 
structed on a common plan.* But the body-segments in the 
Crab are reduced to a minimum size, and merely subserve, in 
the female, as a cradle or marsupium for the eggs before 
hatching, while in the male they are quite rudimentary. In 
the Lobster, on the other hand, the body-segments in both sexes 
are well developed, and serve as a most powerful swimming 
organ, by which the animal can by a sudden jerk propel itself 
through the water with great velocity. 
All the Merostomata have large compound sessile eyes, or, if 
pedunculated, the stalk is immovably fixed to the carapace. 
The larval eye-spots, or ocelli, are usually distinctly seen. The 
appendages are not specialised and set apart for separate duties, 
as in the higher forms of Crustacea, but alike fulfil all the 
functions of locomotion, prehension, and mastication, and for 
this purpose are arranged around the mouth (see Plate XC., 
fig. 1). The branchise and organs of reproduction are borne 
upon a series of lamelliform plates, which are in fact the 
modified pairs of limbs, attached to the under surface of the 
thoracic segments ; there being as many as six plates in Limu - 
lus , five of which are branchiferous, and probably only two, or 
at the most three, in Pterygotus, The remaining segments are 
destitute of appendages. 
In the Eurypterida the body-segments are largely developed 
for swimming (as in the Lobster) ; whilst in the Xiphosura 
they are reduced to a minimum size (as in the Crab). Lastly, 
it is the long-tailed division in each order which is the oldest 
group, the more cephalised type in each being also the more 
modern. 
We have thus seen that Limulus and Pterygotus are as 
intimately related as Crabs and Lobsters are to each other at 
the present day ; but nevertheless the type of Pterygotus is a 
very abnormal type. Let us see what we have like it among 
living forms. 
* See an article on the Lobster, by Mr. St. George Miyarfc, F.L.S., in the 
“ Popular Science Review,” 1888, Vcl. YU. p. 345, pi. xxii. 
