256 DR. A. RATTRAY ON THE ANATOMY, PHYSIOLOGY, 
the semiopaque yellowish buccal and visceral masses; and it is only by removal from 
the water, which renders their tissues semiopaque, that their form is evident, and that 
the intestinal canal, cephalic and pedal ganglia, and other organs, can be traced by 
holding the animal between the eye and the light. But though this renders investigation 
comparatively easy, observers have a serious obstacle to overcome in the difficulty of 
procuring the fresh specimens, and, if possible, living animals, necessary for research, 
which can only be obtained at sea. In spirit and other preservatives the delicate frame- 
work soon becomes shrunk, and the tissues indistinct and scarcely recognizable under 
the microscope. Hence it is that information is still needed on some points, especially 
on the anatomy of the circulatory, reproductive, digestive, and glandular apparatus, 
which under more favourable cireumstances would not be long wanting. And on these 
subjects, therefore, any addition to our knowledge, however trifling, is not without value. 
The following observations, which formed part of the writer's service-journal of H.M.S. 
* Topaze’ in 1861, were made principally on one variety of Firola and one of Firoloides ; 
but, though specially deseriptive, they may be considered generally applicable to the 
Firolidee as a class. | 
Firoloid (Plate XLIV. fig. 19, & XLIII. fig. 1) possesses all the characteristics of the 
family, and very much resembles that sketched in Mrs. Gray's * Molluscous Animals' 
as Firola de Kerandrew (Voy. de la Bonite). 
Firola (Plate XLIV. fig. 17, & XLIII. figs. 3, 4) differs from it chiefly in possessing a 
pointed tail, which does not receive a coat from the external hyaline envelope, and in the 
male generative organs being some distance in front of the branchial nucleus, and an 
the right side. 
Coats of the Firolide. 
These are two in number—an outer transparent Zunic, and an inner semiopaque mus- 
cular envelope. 
The outer hyaloid coat (Plate XLIII. figs. 1-5, A), commencing opposite to the buccal 
nucleus, extends backwards, covers the ventral fin, gradually thickening to about the 
middle of the body, where it again diminishes, and finally blends with the muscular 
coat at the root of the tail. In Firoloides (figs. 4 & 5) it commences at the oral orifice, 
and entirely covers the truncate posterior end. It varies considerably in thickness, and 
sometimes bulges so much centrally as to double the animal’s diameter, making its 
otherwise vermiform shape resemble an elongated cylinder (fig. 1) tapering towards 
either .end ; while in others it is very thin, and sometimes scarcely perceptible. It 
likewise covers the fin, at the free edge of which it is usually well seen even when with 
difficulty detected elsewhere. It also gives a thin coat to the eyes (fig. 8, ay), and 
forms the entire substance of the tentacles (fig. 8, ah), which contain no muscular 
fibres, and are therefore non-retractile, unlike those of the Gasteropods generally. 
Posteriorly it ends without giving a cover either to the genital organs or tail (fig. 1). 
Under the microscope no special strueture can be detected even by a high power, except 
nerve filaments (Plate XLIII. fig. 1, w) traceable from the underlying muscular coat. 
Its surface is equally hyaline ; but in many varieties, both of Firoloides, Firola, and Cari- 
naria, we may observe small round or oval semitransparent cellules; while in others, 
