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Xl. On the Anatomy, Physiology, and Distribution of the Firolide. By ALEXANDER 
RATTRAY, M.D. ( Edin.), Surgeon R.N. Communicated by GEORGE Busk, Esq., Sec. 
L. Soc. | 
(Plates XLIII. & XLIV.) 
Read February 18th, 1869. 
WHILE modern research has entirely revolutionized the previously formed and prin- 
cipally theoretical views as to the laws which govern the distribution of animal life in 
the deeper parts of the ocean, and has proved directly that it can and does exist at far 
greater depths than formerly deemed possible, and indirectly that there are probably few 
spots where it is entirely absent, the fact still holds good that it becomes progressively 
more abundant as we near the surface and approach the vivifying atmosphere. In short, 
it is a region near where the sea and air meet that forms, in both, the principal theatre 
for the development of animal, as it is of vegetable life. Nevertheless, of these two 
great life-sustaining media the former is unquestionably most prolific in animal organi- 
zation, especially in what may be termed microscopie life, with which its surface waters 
in all accessible latitudes, and particularly in tropical regions, literally teem; and the 
towing-net, most successful in calm weather, especially after dark, when the sun's glare 
and heat have declined, readily demonstrates that the beautifully clear and apparently 
uninhabited waters of mid-ocean furnish a habitat for minute beings in such countless 
profusion and variety as to throw its better-known, because more easily noticed, bulkier 
inhabitants completely into the shade. 
Minute crustaceans are seldom absent from the towing-net, and are often met with in 
myriads. Next to them are the Hydrozoa, especially the many and varied forms of Salpa. 
Then come the pelagic mollusca, and among them the shell-less nucleobranchs we are 
now about to consider; but these, in the Pacific at least, are comparatively few in num. 
ber, so that it is only now and again that, in a crowd of jelly-like Salpe, invisible save 
by the tiny currents or jets produced by their peculiar jerking mode of progression, we 
come across a solitary, or at most a few almost equally imperceptible Firolide. But 
the former, though highly interesting in many points of view, unquestionably yield in 
importance to the latter and the Heteropoda as a class, inasmuch as these possess a more 
highly developed nervous system, organs of special sense, and general anatomical struc- 
ture, all indicating a higher place in the scale of creation. 
So pellucid are many of the oceanie mollusca, even those not, strictly speaking, micro- 
scopie, and so delicate their structure, that even the intensely coloured forms cause little 
diminution in the transparency of the deep or change in its hue, except when in shoals. 
The presence of Firoloides, Firola, or Carinaria can be detected with difficulty, except 
in large npocimens, when the eyes become -r as two dark 2. and at either end 
VOL. xxvi. SM. —— 
