120 DESCRIPTIONS OF PREPARATIONS. 



ganglion to the foot The cut ends of the columellar muscles are visible in 

 a bundle below the buccal mass. These muscles are attached on the one 

 hand to the columella of the shell, on the other to the foot, to the buccal 

 mass, tentacles, and nerve-collar on either side. They therefore serve as 

 retractors of these organs. 



There are two pairs of cephalic tentacles in nearly all terrestrial Pulmonata, 

 the superior pair bearing the eyes at their apex : hence the name Stylommatophora 

 applied to this section of the order. They are hollow, and in- and e-vaginable. 

 The exceptions are the Australian genera Janella and Aneitea, in which the eye- 

 bearing tentacles alone are present, and the slugs Onchidium and Vaginulus, in 

 which the tentacles are solid but extremely contractile. The aquatic Pulmonata, 

 on the other hand, have but one pair of tentacles, solid and contractile, with the 

 eyes placed at the inner side of their bases : hence this section of the order is 

 termed Basommatophora. 



The bands of nerve-fibres uniting the various ganglia are termed ' commis- 

 sures ' when they unite the ganglia of the same pair, e.g. the cerebral ; ' connectives ' 

 when they unite ganglia of different pairs, e.g. cerebro-pedal connective between 

 the cerebral and pedal ganglia of the same side. The nervous system of H. pomatia 

 has been worked out in detail by Bohmig. Each cerebral ganglion is divisible into 

 three regions : from one there arise the commissure to the other cerebral ganglion 

 the various connectives, and six nerves, one to the upper tentacle^ the mouth, lip, 

 pharynx, lower tentacle, and on the right side to the penis : from the second arises a 

 nerve to the upper tentacle and eye : from the third nothing. The two pedal ganglia 

 are each divisible into two regions, from which 8-9 nerves pass to the foot, and 

 from one of the ganglia branches proceed to the connective tissue surrounding the 

 uterus. The parieto-splanchnic region consists of a right and left pleural (=com- 

 missural) ganglion which are in union with the connectives to the cerebral and 

 pedal ganglia, and with the remaining ganglia of the visceral system. These are a 

 right and left visceral ganglion and a median abdominal. The right visceral 

 ganglion gives origin to two pallial nerves inclosed in a common sheath ; the left 

 to a single pallial nerve. The abdominal ganglion gives origin to three nerves ; 

 one distributed to the heart, nephridium, liver and (?) sexual organs ; another to the 

 neighbourhood of the anus, and the third to the integument. The buccal ganglia 

 supply the pharynx, intestine and salivary glands with four nerves. Although the 

 divisions between the pedal and the various ganglia of the visceral system are not 

 visible externally in Helix and other terrestrial Pulmonata as they are in the aquatic 

 Pulmonate Ltmnaeus, several facts may be noted which clearly show without further 

 analysis the compound nature of the infra-oesophageal ganglion. These are (i) the 

 distribution of the nerves given off by it ; (2) the passage of the aorta cephalica 

 through its centre ; (3) the relation of the otocysts to the region termed pedal ; (4) 

 the presence of double connectives on each side between it and the cerebral ganglia, 

 which can be clearly discerned through the common connective tissue sheath. 



In Zonites algirus two pedal nerves run backwards in the foot nearly parallel one 

 with another. They are connected by transverse commissures from place to place, 

 and posteriorly they break up into a number of branches which anastomose with 

 one another. Minute ganglia occur at the nodal points of this network, but are 



