PTEROPODA. 



177 



terior (interior) (23, b) is the shortest of 

 the series. The stem upon which these are 

 fixed (22, u>) is sloped off in the same pro- 

 portion, and has a somewhat triangular shape. 

 When crushed under the microscope, it is 

 found to consist entirely of muscular fibres ar- 

 ranged with considerable regularity, and prin- 

 cipally disposed in two opposite directions, so 

 that they cross each other ; and doubtless a 

 part of their office is to raise and depress the 

 individual teeth, implanted upon the common 

 stem. The hinder portion of the cylinders (j') 

 containing this extraordinary dental apparatus, 

 is muscular, and composed of longitudinal fas- 

 ciculi, by the aid of which the stems that sup- 

 port the teeth are retracted, their protrusion 

 being effected apparently by the construction 

 of the capsules themselves. The manner in 

 which the Clio makes use of these teeth may, 

 therefore, be inferred from their anatomical 

 arrangement. The cylinders wherein they are 

 lodged are so much bent (fig. 111. 18, 19, z), 

 that when the two dental organs of the oppo- 

 site sides are protruded the apices of the teeth 

 with which they are armed must meet together 

 outside the mouth, and when in this condition 

 the teeth of each organ are widely separated 

 and spread out, they will form, as it were, a 

 couple of long combs (19, z>), and evidently 

 perform the functions of a pair of tenacious 

 jaws. 



The tongue may be divided into two por- 

 tions ; the one free, and the other fixed, 

 studded with a number of booklets that can 

 scarcely be estimated at fewer than from six 

 to eight hundred, the disposition of which 

 at once indicates their office to be to facilitate 

 the propulsion of the food into the oesophagus, 

 as is the case in the Cephalopods and various 

 other Mollusca. 



The oesophagus is, for the greater part of 

 its length, surrounded by the two salivary 

 glands, which extend quite into the abdominal 

 cavity, where they are connected to each 

 other, and to the liver by lax cellular mem- 

 brane. The stomach is a mere dilation of 

 the oesophagus, and is entirely embedded in 

 the substance of the liver. The latter organ 

 appears, when examined superficially, to be 

 entirely made up of a multitude of Acini, 

 each of which contains within it a cavity that 

 communicates through a wide aperture with 

 the interior of the stomach; and hence it re- 

 sults, that, although the exterior of the liver is 

 seemingly composed of large granules, the 

 walls of the stomach are perforated all over 

 with openings, leading into blind cavities, so as 

 to have a completely cellular appearance. 



The intestine is a simple tube, passing 

 straight from the termination of the stomachal 

 portion of the alimentary canal to the anal 

 orifice, which is situated on the right side of 

 the neck immediately behind the correspond- 

 ing fin. 



The course of the circulation in the Ptero- 

 poda has not been as yet completely made out. 

 In the Clio borealis, the heart enclosed in its 

 pericardium is situated on the right side of the 

 posterior end of the abdominal cavity just at 



VOL. IV. 



the point where the dorsal and ventral bands 

 of muscle separate to form the wide lateral 

 opening. The pericardium is pointed in front 

 and broad behind : its walls are thin and trans- 

 parent, but at the same time very strong. On 

 opening the pericardium, the ventricle of the 

 heart is seen to have the shape of a triangular 

 pyramid with rounded angles, the apex of the 

 pyramid being directed towards the head, 

 whilst its base is turned towards the hind part 

 of the body. From the apex of the heart arises 

 a large vessel, which immediately pierces the 

 pericardium, and supplies branches to the liver 

 and to the internal organs of generation ; it 

 then advances forward, and supplies the parts 

 about the neck, more especially the lateral 

 fins, and most probably is ultimately distri- 

 buted to the head and its appendages. This 

 vessel is evidently the aorta. 



Fig. 113 (25 to 31). 



Clio Borealis. 



25. One of the lingual teeth, magnified 400 di- 

 ameters. 



26. Male generative apparatus, removed from the 

 body and unfolded. 



27. Female generative organs displayed. 



28. Convex surface of the testes. 



29. One of the pigment sacs of the integument, 

 magnified 120 diameters. 



30. Nervous system, magnified 12 diameters. 



31. One of the eyes, magnified 40 diameters. 

 (After Eschricht.) 



Generative system. The reproductive or- 

 gans in the Clio borealis occupy a very con- 

 siderable portion of the abdominal cavity. 



N 



