176 



PTEROPODA. 



are hollow, and that their cavities communi- 

 cate with the common cavity of the head : 

 they have likewise their proper muscles, and 

 each receives a large nerve derived imme- 

 diately from the anterior supra- oesophageal 

 ganglion. As to the use of this elaborate 

 apparatus, there is still room for speculation. 

 Captain Holboll, although he frequently ob- 

 served them porrected, while the creature was 

 swimming, never saw them employed as 

 suckers or instruments of prehension ; never- 

 theless, it seems impossible to doubt that such 

 is their real office, when we reflect upon their 

 remarkable structure, and further take into 

 account their situation, so completely analo- 

 gous to that occupied by the sucking discs of 

 the Cephalopoda, and still more closely re- 

 sembling the cephalic appendages of Pneumo- 

 derma. It is, therefore, extremely probable 

 that these organs are employed for holding to 

 foreign objects at the bottom of the sea, and 

 that the great number of the sucking discs is 

 in correspondence with the power possessed 

 by the Clio of crawling about upon uneven 

 surfaces. 



The mouth of the Clio is a vertical fissure, 

 that is easily displayed by slightly folding back 

 the head-cones (fig. 110. 13, u). Its margins 

 seem to enclose some calcareous substance, 

 which, in specimens preserved in spirit, is of a 

 chalky whiteness. Numerous muscular fasci- 

 culi surround this opening, which, when ex- 

 panded, has somewhat of a triangular form, 

 so that during life the mouth can be forcibly 

 opened by the radiating muscular fasciculi 

 that surround it. 



In the cavity of the mouth there may be 

 observed, on each side, a round fossa, in 

 which can be seen projecting, even with the 

 naked eye, a hard shining substance, first 

 noticed by Pallas and Fabricius, who re- 

 garded these bodies as simple teeth. Closer 

 inspection, however, reveals them to have a 

 very curious structure, which is, perhaps, 

 unique, each consisting of a bundle of about 

 thirty gold-coloured, crooked, stiff and sharp 

 hooks (fig. 112. 22, w} y derived from a 

 common base (#), and forming a pair of 

 lateral jaws, wherewith the creature seizes 

 its food. 



In the middle of the ventral aspect of the 

 cavity of the mouth there is, moreover, a 

 prominent tongue-shaped organ, which, when 

 moderately magnified, may be seen to consist 

 of two lateral bands of a black colour, which 

 are united in the middle line, and which are 

 covered with an immense number of extremely 

 minute teeth, that will be more particularly 

 described hereafter. The pharynx, when ex- 

 amined from above, is somewhat lyre-shaped : 

 it is composed of two lateral branches (fig. 

 111. 19, i), the posterior ends of which are 

 joined by a convex central portion (z). The 

 tube of the oesophagus is not prolonged imme- 

 diately from its hinder extremity, but seems 

 to arise from the hinder wall of the pharyngeal 

 cavity (fig. 111. 17 and 21, e). 



The nerves of the pharynx arise from two 

 ganglia (fig. 111. 18, 20, y) situated imme- 



diately behind it, in conjunction with the an- 

 terior ganglia of the circumcesophageal ring, 

 and which inferiorly are connected together by 

 strong branches of intercommunication and 

 from which nerves radiate laterally to supply 

 the surrounding parts. The thin ducts of the 

 salivary glands (fig. 1 11. 17, 18, 19, and 20, g) 

 terminate above these ganglia opening into the 

 cavity of the mouth in the immediate vicinity 

 of the tongue. 



The pharynx, when viewed with a lens, and 

 still more when examined under the micro- 

 scope, resembles, very closely, the gizzard of a 

 gallinaceous bird, the resemblance consisting 

 in the great strength of its muscular parietes. 



Each lateral portion (fig. 111. 18, t) is a 

 small curved cylinder, the outer wall of which 

 is entirely muscular. The fasciculi are princi- 

 pally arranged in two layers, the fibres crossing 

 each other. On opening one of these muscular 

 capsules, by means of a fine pair of scissors 

 it is found to contain, in its interior, a cylin- 

 drical body made up of several parts. At its 

 anterior extremity are situated the lateral teeth 

 above alluded to (fig. III. 19, and /g.112. 22, 

 v). These are arranged in parallel arches, in 

 such a way that their points all attain the same 

 height, notwithstanding the great difference in 

 their length, the posterior (exterior) tooth (fig. 

 112. 23, a) being far the longest ; while the an- 



Fig. 112(22 to 24). 



Clio JBorealis. 



22, 23 a, 23 b. Dental apparatus, magnified 28 

 diameters. 



24. Lateral view of the free portion of the tongue, 

 magnified 130 diameters. (After Eschricht.) 



