46 PROCERDINGS OF THE MALACOLOGICAL SOCIKTY. 



the body, the highly developed labial palps at the anterior end, the 

 gills at the posterior end, and also, even probably to some extent in 

 primitive forms, the extra sensitiveness of the hinder part of the 

 mantle border. 



The pedal ganglia show a higher grade of specialisation, even in 

 the lowest types of Pelecypods, than in either Amphineura or low 

 Prosobranchs. In the latter cases the ladder-like form, consisting of 

 two longitudinal cords of ganglionic matter iiuited at intervals by 

 commissures, persists, and is no doubt intimately associated with 

 a long creeping sole. In the Pelecypods, one of whose most striking 

 characters is the adaptation of the general body form for digging, the 

 foot has already been specialised as a digijing organ even in the most 

 primitive forms. For this purpose it has assumed a more or less 

 cylindrical shape, with greatly restricted sole area. Corresponding to 

 this change of form, the long pedal cords, which we may assume were 

 present in ancestral forms, became shortened up to form rounded 

 gan<;lia in which occasionally traces of the original ladder - like 

 formation may be found in multiple commissures. 



There is no doubt that the double cerebro pedal connectives found 

 in several of the Protobranchia, unless their posterior roots prove to 

 be the otocystic nerves, constitute a difficulty when comparing the 

 Pelecypod nervous system with one of the Amphineuran type. Were 

 it not for the occurrence of both otocystic nerve and double (though 

 intra-ganglionic) roots to the cerebro-pedal connectives in Solenomya, 

 there would be no question in my mind that Drew's interpretation 

 of the posterior root of the cerebro-pedal connective as the otocystic 

 nerve is correct. But in case further investigation of Solenomya 

 should prove his view to be untenable, I venture to put forward the 

 following suggestion : — May not the posterior root be the vestige of 

 Amphineuran latero-pedal connectives ? These connectives between the 

 pallio-visceral loop and the pedal cords, it will be remembered,' reach 

 their greatest and most characteristic devtslopment in the most generalised 

 types of Polyplacophora {Hanleya, Lepidopleiirus), but as specialisation 

 increases they first become very variable both in position and number, 

 and finally in the higher forms (^Tonicia, Ischnocltiton, Acmitliochiton) 

 vanish. As these connectives are characteristic of the lowest known 

 form of molluscan nervous system, it is no great stretch of imagination 

 to suppose that they also occurred in the forerunners of the Pelecypods, 

 and in this group, as in the Chitons, vanished in proportion to the 

 increase in general specialisation, until at the present day their 

 remains persist in some few Protobranchs as a posterior root to the 

 cerebro-pedal connective. In the light of this suggestion, it is 

 interesting to recall that connections occasionally occur in Pelecypods 

 between the visceral and pedal ganglia.^ 



Whether some such comparison as this, with a nervous system of 

 the present-day Amphineuran type, is justified or not by the facts to 



' Plate, Zool. Jahib. (Fauua Chileusis), Bd. ii (1902), p. 493. 



2 D'Hardvillier, " Sur quel ques f aits qui permittent . . . " : C.R.Ac. Sci., 

 t. cxvii (1893), p. 250. 



