In many places, the cutaneous muscles are still more complex in their arrangement, more particularly in the neighbourhood of the eyes. In the head, and partially also in the neck, where a firmer connexion between the skin and the general muscular strata of the body exists, an expansion of the proper cutaneous muscles is with difficulty to be demonstrated.
The nerves of the integument are easily traced in fresh specimens on account of the transparency of the skin. The most con spicuous are two large cutaneous nerves run ning on each side of the body, which ramify upon its lateral and ventral aspects.
Immediately beneath the skin is a layer of cellular tissue, which is very different in dif ferent regions. In the hinder part of the body it exists in great abundance, and in it, as already stated, the large pigment cells are itnbedded, so that in this region the skin is very easily separated from the muscular strata beneath. It Is most abundant likewise in the region of the heart where the urinary bladder is situated. In the fins, this cellular mem brane is more scanty, and in the regions of the neck and head, it is so dense that here the skin can only be raised with difficulty. In specimens that have been kept in spirits, the subcutaneous cellular tissue is very gene rally infiltrated with fluid so as to give the appearance of a cavity existing beneath the integument, the boundaries of which are cir cumscribed by those parts where the skin is most adherent to the subjacent tissues, or where the cutaneous muscles interlace with each other.
Muscular system.— The muscles of the Clio borealis are chiefly disposed in a single layer, situated beneath the subcutaneous cellular tissue, that encloses the whole hinder part of the body as in a bag, which, however, in the region of the neck and of the head, spreads out into separate fasciculi of muscle. This muscular bag is described by Cuvier* as being composed of very conspicuous longi tudinal fibres, derived from two principal fas ciculi attached to the sides of the neck, the effect of which will be to shorten the whole body, and make it assume a form approxim ating to the spherical. In fresh specimens preserved in spirits, the muscular bag in question is easily visible through the skin ; but in the living animal, it is most likely it self transparent, and in old specimens cannot be seen on account of the opacity of the ex ternal integument. The muscular fibres com posing this sheath, do not by any means run straight and undivided from behind foiward, but, on the contrary, interlace with each other, so as to form an expansion in which the lon gitudinal fibres are the most conspicuous.
From the neck forwards, these muscular bands become more precise in their arrangement. At the sides of the body-, they separate from each other so as to leave a space both be hind and in front,in which the muscular layer is deficient; the dorsal and ventral fasciculi becoming more and more detached as they advance forwards, leaving a wide opening in the muscular sheath, which near the head gives passage to the lateral fin, and behind this for the pair of large cutaneous nerves, also on the right side, close to the fin, for the common opening of the male and female generative ap paratus, and, a little behind the exit of the two cutaneous nerves, for the anus. In its posterior corner lies the pericardium also on the right side but more deeply situated.
These different parts, as they issue through the muscular opening of the right side, are further embraced by muscular fasciculi, which run transversely from the dorsal to the ventral aspect of the body bounding and separating their orifices.
Locomotive Apparatus.—The locomotive ap paratus of the Pteropoda is constructed upon most peculiar principles, consisting of a pair of fin-like expansions attached to each side of the neck of the animal.
These fins, or, as they are commonly called, wings in the Clio have a very remarkable struc ture, the two being continuous with each other, through the intervention of a central part, which extends transversely across the neck of the animal, so that the lateral expansions are only the free extremities of the same organ, the whole apparatus representing, with curious exactness, the double paddle used by the Greenlanders in navigating their light double pointed canoes (Kajaks). The entire appa ratus is muscular, and consists of two layers, precisely similar in their structure, which, at their margins, overlie each other, but are only connected together by means of cellular tissue. The course of the muscular fibres is shown in the annexed figure, representing the whole of the swimming apparatus removed from the body; in which the following parts may be distinguished : — a, the anterior or dorsal margin ; b, the posterior or ventral ex-. cavation ; c c, the posterior, transparent, tri angular lappets which bound the fin ; d d, the posterior outer border ; e e, the posterior inner border ; o, the central portion which traverses the neck ; m nz m, cornmencement of the free portions of the fins.