428 Prof. M'lntosh on the 



second chamber is formed by a loop which adjoins the Literal 

 and inferior part of the former and surrounds the tip of the 

 blood-vessels, as well as inferiorly the oritiin of the lateral 

 muscles. It is from this anterior loop that the chitinous 

 lamella of the lateral expansion originates. By the gradual 

 coalescence of the inner margins of the chitinous rings of 

 the median nuiscles their outline in transverse section 

 resembles a figure of eight; and the lateral loop forms quite 

 as large a chamber appended to each side. Coalescence of 

 the chitinous basement-tissue goes on rapidly, so that a 

 figure resembling a crown is next formed by the investment 

 of the central muscles. Dorsally is a gently convex arch, 

 which dips downward at the median line to join the strong 

 vertical partition, and curves downward externally at the 

 point of junction with the outer wall of the external chamber 

 and the intermediate septum. The latter passes downward 

 and inward to meet the short ventral (chitinous) floor, which 

 in the preparations is slightly drawn upward, where it unites 

 with the vertical septum. The external loop is large and 

 rounded, but is thinner than the investment of the central 

 muscle. It joins the transverse floor at the junction of the 

 external septum, and on each side encloses the external 

 muscle and the special space for the blood-vessels, which lie 

 on the septum between it and the inner chamber. The 

 crown-like arrangement of the chitinous basement-tissue is 

 retained throughout the greater part of the snout, though, by 

 the increase of the lateral chamber and the change in position 

 of the vascular area, the appearances around it are somewhat 

 altered. The median and the lateral septa and the transverse 

 ventral plate are the strongest portions of the chitinous 

 apparatus. The investment of the outer compartment and 

 the loop enclosing the vascular area are thin. 



When the ventral hypodermic tissue in front of the mouth 

 attains considerable depth, the median sulcus of the chitinous 

 crown superiorly is much increased by the shortening of the 

 septum ; and by-aud-by the figure-of-eight shape is again 

 assumed, the lateral septum being almost horizontal 

 throughout the middle third, over the vascular area, which 

 now projects inferiorly in its special chitinous investment. 

 Both chambers are sensibly diminished, and finally the 

 median separate to form with the lateral a figure of eight in 

 transverse section on each side of the central hypoderm. 

 The chitinous ring for the central muscle is irregularly round 

 and has a firm loop for the vessels attached inferiorly, while 

 the outer still gives ofl:' the lateral lamella. Then the 

 chitinous investment of the vessels moves to the inner and 



