Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/671

Rh ISOPLEUROUS GASTROPODS.] MOLLUSCA 643 go eg-- nk- nephridia of Conchifera (organ of Bojanus), the internal aperture lies near the external. From the folded stem of the nephridium very numerous secreting creca 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, p 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. 77* 16). In this respect Chiton is less primi- nq- 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. In Chiton Fl - 15. Dissection of the renal organs (neph- ,. . i i ridia) of Chiton siculus, after Haller (Arbeiten, discrepans, 111 Which Zool. Instit., Vienna, 18S2). F, foot; L, edge of thprp arp GiYfppn r&amp;gt;m r the mantle not removed in the front part of the specimen; s.o., oesophagus; a/, anus; gg, of Ctenidia, the Orifices genital duct; j/o, external opening of the same; ^t 4-Vm m-,Vit.i/4i n r.rv e 9&amp;gt; stem of the nephridium leading to no, its external aperture; nk, reflected portion of the Coincident with the six- nephridial stem; ng, fine cca of the nephri- , . . . . ,. dium, which are seen ramifying transversely teenth pair OI Ctenidia, over the whole inner surface of the pedal mus- those of the genital cularmass &amp;gt;- ducts with a point between the thirteenth and fourteenth ctenidia. In the Neomenise and Chaetoderma 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 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 comparab e to that FIO. 16. Ovary and oviducts presented by the nerve-cords of Ne- of Chiton siculus (after Hal- mp^^n WA T-TVIC onrl r&amp;gt;f flip ArtVirn ler, loc. at.), vb, hb, anterior m 3 worms, ana the ArtJ and posterior suspensor of pod Peripatus. Higher differen- the ovary; U, uterus (en- 7-, ,1 -r 11 i ^ larged part of oviduct); a, nation in other Mollusca leads to oviduct - predominance if not an exclusive presence of nerve : /&res 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 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 Crclo- 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 niUS- Fl - 17.-Diagrams of the nervous system of .. , r. .., Isopleura (alter Hubrecht, loc. at.), c, cere- Clllarity Of the lips and bral ganglia; s, sublingual ganglia; v, pedal nrlnntrmhnrp anrl arp (ventral) nerve-cord; J, visceral (lateral) nerve- oaontopnore, ana are conl. pc&amp;lt; post. anal j unct i on of the visceral found in all G10SSO- nerve-cords. A. Proneomenia. B. Neomenia. i, ,. , v 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. IS.-Anterior part of the nervous Which to compare that of system of Chiton cinerens, in more de- other Mollusca On account of Anatomy). &quot;*& buccal ganglia (con- the small amount of concen- cerned with the odontophore); c, tration of its nerve-ganglio7i cerebral nerve-mass; /, pedal gan- O glion and commencement of pedal cells into ganglia, SUCh as W6 nerve-cord; pi, visceral nerve-cord. fi i 11 f |p V p| f . 1 -.p f ] j n nfTipv The sublingual ganglia are not let- nnd wel1 aev eiopec tered. forms. The development of Neomenia and Chsetoderma from