260 DR. A. RATTRAY ON THE ANATOMY, PHYSIOLOGY, 
animal is enabled to rise at will, or.by expulsion to sink into denser strata,—a beautiful 
adaptation in both of means to ends. Such an arrangement almost necessitates the pre- 
sence of some opening or apparatus more under the control of the animal than the 
superficial tubercles already described possibly can be, inasmuch as no muscular fibres 
or contractile tissue can be noticed, to aid in opening or shutting their orifices, and thus | 
regulate the entrance or exit of the surrounding fluid. And this appears more likely to 
be found in or near the buccal cavity or cesophagus, where the muscular coat is most 
developed and complete, or perhaps, as with the corresponding aquiferous system in 
the Holothuriæ, near the orifice of the vent, than in any other part of the animal. 
Maedonald * has described a small oval opening in the right side of the visceral mass of 
Firoloides and immediately in front of the rectum, which is surrounded by a little sphincter 
muscle intersected by numerous radiating fibres, by which frequent rhythmical contrac- 
tions and dilatations are produced. Probably this may be an opening of the above- 
indicated nature—a supposition supported by the fact that it is in the Firoloides, in which 
the branchiæ are absent, that this is most needed, to effect a rapid change in the blood- 
oxygenating medium. A communication, however, between these and the proper blood- 
vessels within the muscular coat, and the constant or frequent admixture of the blood 
with salt water, appears, on calm reflection, a very unphysiological and untenable sup- 
position. 
Nervous System. 
This, like the entire animal, is symmetrical, and consists of certain ganglionie centres, 
all bilobed, indicating their double nature, arranged along the median line, and united 
by well-developed nerves of intercommunication. The symmetrical character of the 
Firolidæ is even better seen in the arrangement of their nervous system than in their 
external configuration ; and, with the exception of the long interspace which separates 
the ganglia, a great similarity is apparent between it and the nervous system of certain 
classes of the Articulata, e. g. the Insecta—a resemblance further strengthened by the com- 
paratively high development of the organs of special sense, especially vision. In short, 
though a mollusk and non-articulate, the nervous system is more allied to that of the 
Homogangliata than to the Heterogangliata. There are four ganglionic masses, viz. the 
buccal, cephalic, pedal, and branchial, all of which lie, with their intercommunicating 
nerve-cords, close to, and sometimes even embrace the alimentary canal (Plate XLIII. 
figs. 1-5), which runs, generally speaking, along the median axis. | 
1st. The buccal ganglion (Plate XLIII. fig. 5, a) is the smallest, and lies above the 
esophagus, close behind the buccal mass, of which it forms part. It is nearly round ; 
but a slight antero-posterior constrietion marks its double character. Anteriorly it 
sends off two chords (fig. 5, a’), doubtless principally motor, which run forward, sub- 
divide, and enter the buecal mass to end in the tongue, front of the cesophagus, and 
muscular tissue near the mouth; while posteriorly other two (fig. 5, b,c) run upward, 
then backward, to embrace the cesophagus and join two larger, one on either side, from 
the cephalic ganglion, to form the long so-called “ œsophageal collar" (Owen). From 
this interganglionic cord twigs are given off (fig. 5, d) to supply the muscular envelope 
A * Transactions of the Royal Society of Edinburgh, vol. xxiii. part 1. RR 
