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