NATURAI. 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 endostemites 

 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 endostemites of each side, whUe 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. xxxiir and xxxiv). 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 wUl 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 part of the body with the delicate gills can be withdrawn with impunity.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 

 endostemites, 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 endostemite to 

 which are attached certaui 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 rostmm 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. 



