44 
PKOFESSOR H. N. MOSELEY. 
directly without branching as separate strings from the plane 
of ramification to the megalsesthetes. This is the case in 
Acanthopleura spiniger (PL VI, fig. 6) ; in other forms 
larger primary branches arise from the ramifications, and, taking 
a course vertical to the surface, give off the strands leaving the 
megalaesthetes on secondary and tertiary branches (PI. VI, 
fig. 8). The mode of ramification is probably dependent on the 
thickness of the tegmentum. The macraesthetes where fully deve- 
loped, as, for example, in Acanthopleura spiniger, are more 
or less fusiform bodies which occupy the cavities of the mega- 
lopores. Externally at the mouths of the pores they terminate 
in obconical or somewhat dice-box shaped plugs of transparent 
highly refracting tissue, which are extremely conspicuous when 
the decalcified tegmentum is viewed from the outer surface 
under the microscope. Internally their bodies are directly 
continuous with their respective strands of soft tissue (PI. VI, 
fig. 6, a, p). The bodies of the megalaesthetes are composed 
of a number of cylindrical strands of tissue held closely 
together so as to form a bundle which, on transverse section, 
shows the component strands cut across without indication of 
any definite concentric arrangement, Some of the strands show 
a transverse situation, whilst others are not striated. They bear 
nuclei at intervals. I have not been able to examine these 
structures in living specimens, or such as have been specially 
prepared for histological examination, and therefore am uncer- 
tain as to the details. Van Bemmelen (1. c., fig. 11) has figured 
a megalsesthete of Chiton marginatus, giving histological 
details of the body of the organ, which are, I feel sure, more 
correct than mine. 
The terminal knobs, however, of all the megalaesthetes which 
I have examined, except, perhaps, in Chitonellus, show a more 
complicated structure than van Bemmelen represents in C. 
magnificus. All the terminal knobs terminate in a flat disc. 
This disc shows, on careful focussing, a series of concentric rings 
(PI. V, fig. 8, a), as if composed of a series of concentric layers or 
inverted cones fitted one within the other. Further, the neck of 
the inverted cone or dice-box forming the knob shows a series 
