NATURAL HISTORY OF AMERICAN LOBSTER. 
221 
which thus arises by the repetition of simple units on the ventral side of the thorax 
becomes more complex through the divergence and coalescence of both endosternites 
and endopleurites at a higher level to form an archway for the sternal sinus. The roof 
of this passage is discontinuous, being formed by the fusion on the midline of the inner 
processes or mesophragms of the endosternites of each side, while their outer processes 
or paraphragms unite with corresponding horizontal plates of the endopleurites. 
The endophragmal skeleton greatly increases the area for the attachment of muscles, 
and serves to bind the somites of the cephalo-thorax together with greater rigidity, as 
well as to protect important organs, for not only does the archway securely lodge the 
large blood sinus, but it also gives passage to the nerve-cord, access to which from above 
can not be had without cutting through its roof (pi. xxxiii and xxxrv). Since, as is 
well known, this linkwork is shed in one piece, how do the central nervous system and the 
parts adjacent to it escape unharmed? I have never heard this simple question raised, 
but the answer is given by the molted shell, in which it will be seen that the roof of the 
archway is completely absorbed as well as a large part of the intersegmental and dividing 
partitions of the bulkheads referred to above, so that the whole under surface of this 
part of the body with the delicate gills can be withdrawn with impunity. 
The endophragmal skeleton 'bears the hinges for the articulation of the limbs, 
the arrangement of which is peculiar (pi. xxxvii and xxxviii). The central hinges 
which lie close to the mid-line are all cups and are borne on the sterna and close to the 
endosternites, while the outer or peripheral hinges are all balls and are borne on the 
epimeral surface of the branchial cavity, close to the endopleurites. The transverse 
partitions are parallel with the axes of articulation of the appendages in successive somites. 
The hard skeleton of the lobster’s head immediately in front of the mouth, repre- 
senting apparently the sterna of somites ii to iv, consists of a conspicuous plate shaped 
like an Indian arrowhead or spear, with the point drawn out into a sharp spine lying 
between the first segments of the lesser antennae, while its broad base, raised into a 
ridge, bears the soft upper lip or labrum; immediately in front of the ridge this triangular 
plate is traversed by a deep furrow, in the midst of which lies a small closed pit, most 
obvious in a soft-shelled animal. This marks the position of a median endosternite to 
which are attached certain small muscles leading ventrally to the esophagus and 
dorsally to the membranous covering of the brain. 
Upon examining the skeleton of the head from the inside, it is seen that the epimeral 
and tergal parts are fused to form a ring into which the eye stalks open, close to the 
brain. On the upper side at the base of the rostrum the ring forms a solid bar, which 
Professor Huxley thought might represent the tergum of the antennulary somite in the 
crayfish, and from either side of this bar spring two large leaf-like divergent plates, 
the procephalic processes, to which the anterior gastric muscles are attached. Below 
the ring the calcified epimeral surface surrounds the large paired openings for the anten- 
nules and antennae, and is continued to form the wall of the branchial chamber on 
either hand. 
