AND CLASSIFICATION OF THE HETEROPODA. 5 



The whole length of the animal did not exceed an inch and a quarter ; and 

 when placed under the microscope, the principal features of its anatomy were 

 traceable through the integuments. (Plate I. figs. 1, 2, 3 and 4.) 



The head supported two delicate tentaculoe of moderate length and taper form , 

 and a large and beautiful eye occupied a considerable dilatation at the back part 

 of the base of each. The muzzle was rather slender, with a small truncated ex- 

 tremity, within which was contained a minute buccal mass, lodging a short 

 lingual sac. At the posterior part of this organ, the buccal artery terminated 

 between two nodules of nervous matter {haccal ganglia) (Plate I. fig. 2, d), while 

 a narrow oesophagus proceeded from its upper and fore parts. 



The principal viscera were clustered together at the hinder extremity of the 

 body, which is abruptly truncated above, but produced inferiorly into a suddenly 

 tapering tail (Plate I. fig. 4, g), terminating in a filamentous appendage (Plate I. fig. 

 1, n, fig. 4, h), marked at certain intervals by slightly dilated joints or rings, softly 

 tinted with brownish pigment. This appendage is highly sensitive and mobile, 

 trailing in an undulatory manner after the animal as it swims. I can safely 

 say that it is neither a tapeworm nor an oviduct, though I am as yet unac- 

 quainted with its use. 



The characteristic laterally compressed and fan-like foot (Plate I. fig. 1, g) 

 of the Heteropod crested the ventral surface of the body at a little distance poste- 

 rior to the centre, and near the fore part of the free margin of this organ a minute 

 sucker-disc (Plate I. fig. 1, f) was distinctly visible. 



The body in this species is everywhere enveloped by a perfectly transparent 

 homogeneous-looking integument, the inner surface of which is studded at 

 irregular intervals with small clusters of cells, surrounding a larger vesicle (most 

 probably cutaneous glands). Internal to this there is a stout muscular sheath 

 (Plate I. fig. 2, o), which includes the buccal mass in front, and the visceral nucleus 

 behind where it extends into the tail, the homologue of the operculigerous lobe 

 of the foot in ordinary gasteropods. 



The preceding sketch will suffice to give a general idea of the animal ; but we 

 shall now proceed to consider more in detail the anatomy of the organs of sense 

 and the several organic systems, which will enable us to understand more 

 thoroughly the modifications of this type occurring in the other genera. 



Organs of Sense and Nervous System. — The eyes of Firoloides (Plate I. fig. 1, b, 

 fig. 2, 1), like those of other Heteropoda, are very large and beautiful, and situate at 

 the posterior part of the base of the slender tentacula. They are invested by the 

 common integument, and exhibit a very remarkable structure. A little bulb, some- 

 what compressed from above downwards and constricted in front, forms the body 

 of each. This is usually of a pale reddish-brown tint, and sparingly lined with 

 darker pigment granules. A perfectly spherical lens of highly refracting material 

 rests in a depression at its fore-part, and the lens itself is capped over by a strongly 



VOL. XXIII. PART I. B 



