106 



MOLLUSCA 



S.O.. 



F- 



nephridia of Conchifera (organ of Bojanus), the internal 

 aperture lies near the external. From the folded stem of 

 the nephridium very numerous secreting cseca are given off, 

 omitted in the dia- 

 gram (fig. 14, D), but 

 accurately drawn in 

 fig. 15. The sexes in 

 Chiton are distinct, 

 and the ovary or testis, 

 as the case may be, 

 though lying in and 

 filling a chamber of 

 the original ccelom, 

 does not discharge into 

 the pericardium, but 

 has its own ducts, 

 which pass to the ex- 

 terior just in front of 

 those of the nephridia 

 (fig. 14, D, g, and fig. ' nt- 

 16). In this respect 

 Chiton is less primi- 

 tive than the other Iso- 

 pleura, and even than 

 some other Gastropods 

 (the Zygobranchia), 

 and some Conchifera 

 (Spondylus, &c.), which 

 have no special genital 

 apertures, but make use 

 of the nephridia for 



this purpose. InChifon F '- 15. Dissection of the renal organs (neph- 

 , . , ndia) of Chiton siculus, after Haller (Arbeiten, 



dtscrepans, in which 



there are sixteen pairs 



of ctenidia, the orifices 



of the nephridia are 



coincident with the six- 

 teenth pair of ctenidia, 



those of the genital 



ducts with a point between the thirteenth and fourteenth 



ctenidia. 



In the Neomenise and Cheetoderma the nephridia are 

 short and wide (N in fig. 14, A, 

 B, C), and function as excretory 

 ducts for the genital products, the 

 gonads being lodged in the long 

 pericardium. Their separate or 

 united apertures open near the anus 

 into the small chamber formed by 

 the restriction of the mantle-skirt 

 to the immediate neighbourhood of 

 the anus. 



The nervous system of the Gas- 

 tropoda Isopleura is represented in 

 the diagram fig. 17. In all it is 

 important to observe that nerve- 

 ganglion cells are by no means 

 _ a limited .to special swellings the 

 ganglia but are abundant along 

 the whole course of the four great 

 longitudinal trunks. This is a pri- 

 mitive character comparable to that 



FIO. 16. Ovary and oviducts presented by the nerve-cords of Ne- 

 i*>r inr dt ^ h fift anterior niertine worms, and oi tne -A.rt.uro- 

 and posterior s'usp'ensor of pod Peripatus. Higher differen- 

 iarged V part of" oviduct) [oj tiation in other Mollusca leads to 

 oviduct predominance if not an exclusive 



presence of nerve-fibres in the cords, and of nerve-ganglion 



cells in the specialized ganglia. The numerous transverse 



connexions of the pedal nerve-cords in Chiton and Neo- 



Zool. Instit., Vienna, 18S2). F, foot ; L, edge of 

 the mantle not removed in the front part of 

 the specimen ; s.o., oesophagus ; a/, anus ; gg, 

 genital duct ; go, external opening of the same ; 

 eg, stem of the nephridium leading to no, its 

 external aperture ; nk, reflected portion of the 

 nephridial stem ; ng, tine cseca of the nephri- 

 dium, which are seen ramifying transversely 

 over the whole inner surface of the pedal mus- 

 cular mass. 



menia (seen also in Fissurella (fig. 36) and some other 

 Gastropods) are comparable to the transverse connexions 

 of the ventral nerve- . 



cords of Chsetopod 

 worms and Arthro- 

 pods. In the abund- 

 ance of the nervous 

 network connected 

 with its longitudinal 

 nerve-tracts, Chiton 

 appears to retain some- 

 thing of the early con- 

 dition of the Coelo- 

 mate nervous system 

 when it had the form 

 of a sub-epidermic net- 

 work or nerve -tunic 

 (seen more clearly in 

 Planarians and some 

 Nemertines), and when 

 the concentration into 

 definitely compacted 

 cords had not set in. 



Ganglia are, how- 

 ever, distinguishable 

 upon the nervous cords 

 of Chiton (fig. 18). The 

 cerebral ganglia are 

 not distinguishable as 

 such, but a pair of 

 buccal ganglia (B in 

 fig. 18) are developed 

 on two connectives 

 which pass forward 

 from the cerebral re- 

 gion to the great mus- 

 cular mass of the 

 mouth. These buccal 

 ganglia are special de- 

 velopments connected 



with the snprial mils- Fl - 17. Diagrams of the nervous system of 



Wlin me sp Isopleura (after Hubrecht, loc. eit.). c, cere- 



CUiarity OI tne lips and bral ganglia ; s, sublingual ganglia ; v, pedal 



nrlnnrrmVinrp flnri nrp (ventral) nerve-cord ; I, visceral (lateral) nerve- 



lontopnore, ana are cord . p ^ pos t-anal junction of the visceral 



found in all G10SSO- nerve-cords. A. Proneomenia. B. Neomeuia. 



. , , i C. Chsetoderma. D. Chiton. 



phora, but not in the 



Lipocephala. Such special ganglia related to special 

 organs (and not introduced in our schematic Mollusc, fig. 



1) we find in connexion with 

 the siphons of the Lipoce- 

 phala, and in various posi- 

 tions upon the visceral nerve- 

 cords of other Mollusca, both 

 Glossophora and Lipocephala. 

 A pair of pedal ganglia but 

 little developed (p in fig. 18), 

 and a special group of sub- 

 lingual ganglia are present in 

 Chiton. On the whole, the 

 nervous system of the Iso- 

 pleura is exceedingly simple 

 and archaic, whilst it does not 

 well serve as a type with 



FIG. Ik-Anterior parf of the nervous which to compare that of 

 system of Chiton citiereus, in more de- other Mollusca On account of 

 tail(fromGejrpnbaur, KlementsofComv- .1 n < 



Anatomy). B, buccal ganglia (con- the small amount of concen- 

 cerned with the odontophore) ; c, tration of its nerve-ganglion 



cerebral nerve-mass; P, pedal gan- . , 



glion and commencement of pedal cells into ganglia, SUCil as We 

 nerve-cord; pi, visceral nerve-cord. fl j 11 Jpvpl-,! ; n nHipr 

 Tlie sublingual ganglia are not let- nnd wel1 developed in OUier 

 tered. forms. 



The development of Neomenia and Chaitoderma from 



