172 



PTEROPODA. 



substance, muscuiar fasciculi are perceptible, 

 the direction of which is principally towards 

 the crucial muscles of the fins. Upon the 

 dorsal region of the body, these tegumentary 

 muscles first become distinct at the trans- 

 verse constrictions above referred to. These 

 constrictions disappear as soon as the skin 

 is cut through, and the inner layers of the 

 dorsal region then appear quite lax. In this 

 way, indeed, the existence of transverse fas- 

 ciculi of cutaneous muscles is rendered evi- 

 dent, even when their presence cannot be 

 proved by direct observation. 



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. 



Fig. 108 (1 to 6). 



1. Clio Boreulis, seen from the ventral aspect, the 

 head-cowls shut together. 



2. The same ; the head-cowls turned back, and 

 the cephalic and generative apparatus displayed. 



3. Details of ditto. 



4. Clio borealis with the head cowls closed, seen 

 from the dorsal aspect. 



5. Side view (right) of the same, the fins cut off 

 at their roots. 



6. Details of ditto. (After Escliricht.') 



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 

 imbedded, 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 forward, 

 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 



* Me'moire sur le Clio, p. 6. 



