174 



PTEROPODA. 



superficially, either from the dorsal or ventral 

 aspect, they have the appearance of forming 

 one elongated mass. By means of a nervous 

 band which connects them, the eight ganglia 

 form a double ring, seeing that the two lateral 

 pairs of ganglia are, as well as the inferior, 

 brought into communication with each other 

 through the intervention of a cross branch 

 which runs beneath the oesophagus. 



In addition to the eight ganglia above men- 

 tioned there are likewise two small nervous 

 masses (Jig. 113. 30, 5), situated one on each 

 side of the anterior pair, with which they are 

 connected by short nervous branches. 



All the nerves given off from these centres 

 seem to proceed from the ganglia nearest to 

 their place of destination. From the anterior 

 pair are derived all the nerves supplying the 

 parts of the head and the eyes. From the 

 lateral pairs the nerves of the fins are prin- 

 cipally given off, while the posterior pair fur- 

 nishes nerves to all the hinder parts of the 

 body. 



Eyes. The eyes in Clio are situated upon 

 the dorsal aspect of the body, in the constric- 

 tion which constitutes the neck. In this 

 situation the skin is drawn deeply around 

 them, so that they seem to be lodged in spe- 

 cial depressions appropriated to receive them. 

 Each eye (fig.113. 31) has somewhat the shape 

 of a bent cylinder, the two ends of which are of 

 a spherical form. The external spherical ex- 

 tremity of this eye, which is transparent, and 

 constitutes the cornea, stands prominently 

 above the level of the skin. By transmitted 

 light it is not difficult to distinguish the con- 

 struction of the interior. The middle third of 

 the cylinder is generally of a dark colour, whilst 

 the anterior and posterior extremities of the 

 cylinder are comparatively transparent ; but, 

 probably, in the recent animal, the dark pig- 

 ment extends back as far as the hinder end 

 anteriorly, it is easy to perceive the existence 

 of a transparent lens ; but from the small size 

 of the organ, it is difficult to make out their 

 structure more completely. In connexion 

 with these eyes, delicate muscular fasciculi 

 may be traced radiating in different directions, 

 which would seem to have the office of 

 turning the eye-ball towards any particular 

 object. 



The only other special organs of sensation 

 possessed by the Clio are the tentacula; but 

 these will be best described in connexion with 

 the head to which they are appended. 



Head-cowls and Tentacula. The structure 

 of the head of Clio is very remarkable; and, 

 in its general characters, cannot be more ap- 

 propriately described than in the words of 

 Pallas.* " Caput contractual subgloboso- 

 didymum est, lobo vel utroque vel alterutro, 

 imo quandoque neutro, antice papilla carnea 

 (tentaculum) acuta, mucronata. Qui lobi sunt 

 proprie praeputia duo (the head-cowls') crassa, 

 carnea ; hemisphasrica, contractilia, basi coad- 

 unata, e quorum interiore latere emergunt 

 tentacula (head-cones) tria carnosa, conica, 



* Spicelegia, x. p. 28. 



asqualia quae ori utrinque adstant et contracta 

 in praeputio tola delitescunt." 



Fig. 110(8 to 13). 

 8 10 



Anatomy of Clio. 



9. Transverse section of the ventral fasciculi, as 

 they pass through the nerve surrounded by the mus- 

 cular collar (v). 



10. Head of Clio, with the cowls half expanded, 

 showing the conical cephalic appendages (s), and 

 one of the tentacula (k) protruded. 



11. Head of Clio, cowls closed, and the left ten- 

 tacle protruded. 



12. The same seen from above. 



13. The same, the cowls being widely separated 

 so as to display the opening of the mouth. 



(After Eschricht.) 



The above description will, however, be 

 better understood by a reference to the ac- 

 companying figures, in which the structures 

 above mentioned are delineated on a large scale. 

 In fig. 110. 11 the head is represented, seen 

 from the ventral aspect with the head-cowls 

 (a, b) closed together, concealing all the other 

 organs except the tentacula, one of which () 

 is seen protruding through an opening in the 

 left cowl, that of the opposite side being re- 

 tracted while in fig. 110. 10 the head-cowls 

 are shown partially folded back, so as to dis- 

 play the conical appendages (head-cones) 

 which the cowls enclose and protect. 



Each of the cowls (lobi, Pallas ; buccts, 

 Fabricius) seems, when more closely ex- 

 amined, to be composed of two spherical parts 

 intimately conjoined, of which the anterior (fig. 

 110. II, a a) is the smaller, and the posterior 

 (b b) the larger. The posterior spherical por- 

 tions are continuous with each other ; they 

 enclose a large cavity, which is, in its widest 

 part, filled up by the penis ; but, in its nar- 

 rower and median part, contains the parts of 

 the mouth the oesophagus and the salivary 

 apparatus. The smaller or anterior spheres, 

 on the contrary, are only produced by the 

 folding of the skin over the head-cones, and 

 disappear when these organs are protruded. 

 In the fore part of each of the anterior sphe- 

 rical portions of the cowl is a little flat sur- 

 face, in the middle of which may be observed 



