106 



MOLLUSCA 



nepkridia of Conchifera (organ of Bojanus), the internal 



aperture lies near the external. From the folded stem of 



the nephridiiim 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 coelom, 



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. 



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, 



diserepans, 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 Chsetoderma 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 Gfas- 

 tropoda Isopleura is represented in 

 the diagram fig. 17. In all it is 

 important to observe that nerve- 

 ganglion cells are by no means 

 limited to special swellings — the 

 gangUa — but are abundant along 

 the whole course of the four great 

 longitudinal trunks. This is a pri- 

 mitive character comparable to that 



Fig. 16.— Ovary and ovidnots presented by the nerve-cords of Ne- 

 of CM(o»sicui«s (after Hal- mertine worms, and of the Arthro- 



lei, loc. cn,t.). rJ, M, anterior , -r, ■ , -rx- i t-n- 



and posterior suspensor of pod Feripatus. Higher dmeren- 



tged"^ o^'o^rot);^ttiation in other MoUusca leadsto 



oviduct. predominance if not an exclusive 



presence of nerve-^6rc^ in the cords, and of n&^y^ganglion 



cells in the specialized ganglia. The numerous transverse 



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



In Chiton ^i"- l^* — Dissection of the renal organs (neph- 

 ridia) of Chiton sicuZus, after Haller iArbeiten, 

 Zool. Institj Vienna, 18S2). i^, foot ; L, edge of 

 the mantle not removed in the front part of 

 the specimen ; s.o.j oesophagus ; af, anus ; ffgr, 

 genital duct ; go, extern^ opening of the same ; 

 eg, stem of the nephridium leading to 7W, its 

 external aperture ; nk, reflected portion of the 

 nephridial stem ; ng, tine cseca of the nephri- 

 dium, ■which axe seen rami^ng transversely 

 over the whole inner surface of Uie pedal mus- 

 cular mass. 



A 



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

 Gastropods) are comparable to the transverse connexions 

 of the ventral nerve- . j. 



cords of Chaetopod 

 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 special mus-l''?- IT.-Diagams of the nervous system of 



1 • i r i_ 1 • J Isopleura (after Hubrecht, loe. ciJ.). c, cere- 



CUlarity OI tne lips and tral ganglia ; s, subungual ganglia ; v, pedal 



nrInntr>Ti>inre nnrl arp (ventral) nerve-cord ; Z, visceral (lateral) nerve- 



oaontopnore, ana are ^^^^. ^^ post-anal junction of the visceral 



found in all GIOSSO- nerve-cords. A. Proneomenia. B. Neomenia. 



■I 1 . . • . -i C. Chaetoderma. D. Chiton. 



phora, but not m 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 MoUusca, 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 



u 



pc 



which to 



that of 



Fig. 18.-Anterior part of the nervous wmcu uo compare 

 systemof CMtomcinereM, inmorede- other MoUusca on account of 

 tail(fromGegenbaur,jEte7)i«nteo/Comj). ,, ,. j. r 



Anatomy). B, buccal ganglia (con- tne Small amOUnt 01 COncen- 



cerned with the odontophore) ; c, t^ation of its nerve-sanglion 



cerebral nerve-mass; P, pedal gan- "'"'""" "'■ •■'"' "^-i • v^g""5""u 

 glion and commencement of pedal cells mto ganglia, SUCh aS we 

 nerve-cord: pi. visceral nerve-cord. £„ j ,„„ii j„„„i j ;« ^iA 



The subiingiSii ganglia aie not let- hnd well developed m Other 

 ter«d. forms. 



The ^development of Neomenia and Chaatoderma from 



