MOLLUSCA. 



MOLLUSCA. 



870 



of the molluscan kidney is the deposition of a quantity of urinary ex- 

 cretion beneath a free surface, which in all aquatic Mollusca, is, by 

 some means or other, freely bathed with water. In Phallwgia, for 

 instance, minute rounded sacs, each clothed with a delicate epithelium, 

 and containing one or many concretions, are scattered over the intes- 

 tine immediately beneath the lining of the atrial cavity. It is 

 probable that the constant current setting through this cavity carries 

 away some portion of the secretion ; but the greater part seems to 

 remain, and eventually coats the whole parietal surface of the atrium. 

 Here the secreting part of the apparatus appears to be out of pro- 

 portion to the excretory. In the Pteropoda and Heteropoda the 

 reverse relation would appear to obtain. In these animals in fact the 

 concretions have not yet been detected ; but the excretory apparatus 

 is an elongated sac which opens at one end by the side of the anus, 

 and at the other communicates with the pericardial blood-sinus. The 

 sac contracts rhythmically and with great rapidity, so that the blood 

 in contact with its delicate walls must be very effectually washed. 

 How far the internal communication with the blood-sinuses is avail- 

 able for the same end, is not at present understood. In the Laimlli- 

 branchiata, (at least in Unio) the pericardial sinus is connected 

 anteriorly with the internal cavities of two spongy bodies the glands 

 of Bojanus in which a great quantity of concretionary matter may 

 be detected; on the other hand, the outer surfaces of these glands 

 lie in a cavity which admits the water freely by an opening placed 

 anteriorly close to the genital aperture. This cavity clearly corresponds 

 witli the contractile sac of the Pteropoda and Heteropoda, but no 

 evidence of contractility has yet been observed in it or in the renal 

 organ iUelf. Keber also denies that any direct communication exists 

 between the interior of the kidneys and pericardial sinus and the outer 

 sac, but it is somewhat difficult to make sure of this. However this 

 may be, the arrangement of the kidney in Vnio is very interesting from 

 ita close analogy with what obtains in the Cephalopoda, where the 

 ' serous cavities,' which open at the base of the gills and contain 

 the peculiar spongy venous appendages attached to one of their walls, 

 correspond exactly with the excretory sacs of the Lamellibranchiata, 

 while the spongy appendages themselves are but the glands of Bojanus 

 in another form. Our limits will not permit of the description of the 

 structure of the renal organ in A'udioranchiata and Pectinibranchiata, 

 but it might readily be shown to resemble in all essential points that 

 of the Lamcllibranchiata and Cephalopoda. 



5. The Nervous System of the Mollusca. The Mottuscoida and the 

 Molliuca respectively present a remarkable agreement in the general 



Fig. 11. 



Diagram* of the Central Nervons System 1, WaMrimia ; 2, Phalluaa ; 3, 

 fjimfllibrant fiiata ; 4, Pteroeerag ; 5, Atlanta; 6, Firola ; 7, Patella; 8, 

 Dullaa ; 3, Eoli (aflcr Alder and Hancock); 10, Crileil ; 11, Ommaitrcplies 

 (Hancock) ; 12, Nautilta (Owen). The circle* with central dots represent the 

 indltory resides. 



ff, ciliated sac ; r, cerebral ganglia j y, pedal ganglia ; z, paricto-splanchnic 

 ganglia. 



arrangement of their nervous apparatus, which consists in the Polyzoa 

 and Ascidioida of a single ganglion placed in the midst of the neural 

 region of the body ; in the former case between the oral and anal 

 apertures, in the latter between the oral and cloacal apertures. In 

 the Brachiopoda the nature of the nervoua system is only known with 

 certainty in the Terebratulidce, where it consists of a single elongated 

 ganglion having the same position as in the Polyzoa, sending on each 

 side a commissural branch to surround the inouth, and giving off 

 numerous branches to the mantle. In the Brachiopoda no distinct 

 organs of sense have yet been observed, but in the Hippocrepian 

 Polyzoa a little tongue-shaped organ projecting from the lophophore 

 close to the ganglion, probably represents the 'languet' of the Ascidi- 

 ans, an organ whose function is not known, but which probably 

 performs, in conjunction with the ciliated sac, the part of an organ of 

 sense. The 'ciliated sac' is, as its name implies, essentially a small 

 ciliated pouch placed between the oral end of the hypopharyngeal 

 band and the circlet of tentacles. In the Cynthitt, Phallus'u?, &c., 

 it becomes enlarged and twisted upon itself, so that its margin 

 frequently presents a very elegantly convoluted pattern, fy. 11, 2, cs. 

 In this form it was described by Savigny as the ' Tubercule AnteYieuro'.' 

 In Appendicularia and in the Salpce an otolithic sac is also attached 

 to the ganglion. 



In all the Mollusca proper the nervous system presents a remarkable 

 uniformity as to its central elements, and remarkable differences in 

 their arrangement. There are essentially three pair of ganglia : 



1. The Cerebral, which supply the eyes and olfactory organ, and 

 give off the nerves to the buccal ganglia where they exist. 



2. The Pedal Ganglia, which supply the foot with nerves, and 

 always, save in Heteropoda and perhaps some Nudibranchs (where 

 the exception is very possibly only apparent), give off the nerves to the 

 auditory vesicles. 



3. The Parieto-Splanchnic Ganglia, which supply the haemal region 

 of the body and many of the viscera. 



There are never more than two pedal and two cerebral ganglia, but 

 the parieto-splanchnic centres would seem to be capable of almost 

 indefinite multiplication. These multiplied centres however may be 

 reduced to two classes Parietal Ganglia, which give nerves to the 

 sides of the body, and Visceral Ganglia, which supply the heart, 

 branchiae, itc. 



The accompanying diagramatic figures of the nervous systems of 

 Molluscs of all classes, in which the Cerebral Ganglia are marked x, 

 the Pedal y, and the Parieto-Splanchnic z, will render the great changes 

 of position, while the essential parts remain the same, obvious without 

 further description. 



For the organs of sense of the Mollusca proper we must refer to the 

 articles CONCHIFERA, GASTEROPODA, &c. 



4. The Development of the Mollutca. Those conceptions which the 

 philosophical anatomist comprehends under the name of Archetypes, 

 or Common Plans of Animal Forms, must always present a certain 

 value and interest to all who regard anatomy as something more than 

 an exercise of the memory ; but the amount of the value of such con- 

 ceptions, and of their beneficial influence on the forward progress of 

 science, depends entirely on the extent to which they embrace the 

 whole anatomical peculiarities of a group of animals. Now animals, 

 like all living beings, not only are, but become ; and their anatomy, in 

 the widest sense of the term, is to be obtained, not merely by tho 

 study of their structure (which is their final anatomy), but also by 

 that of their development, which is the anatomy of the successive 

 states through which they pass in attaining their final condition. 

 Now the Archetype or Common Plan professing to be the embodiment 

 of the most general propositions which can be enunciated with regard 

 to the anatomy of the group, ita validity will depend upon its em- 

 bracing both structural and developmental facts. If it neglect either of 

 these, it will be theoretically imperfect, and will run the risk, at any 

 rate, of t eing practically erroneous. Before the publication of Von Biir's 

 great work, and unfortunately too often since then, the extant notions 

 of archetypes, unity of organisation, &c., were open to precisely this 

 objection, their authors having contented themselves with devising 

 hypotheses to fit the facts of adult structure, without concerning them- 

 selves whether their hypotheses would or would not also fit the facts 

 of development. Hence the infinite variety of baseless speculations of 

 the ' Nature-philosophie ' school; in botany, the unlimited and quite 

 gratuitous demands upon 'abortion and fusion" of parts which 

 Schleiden has so justly ridiculed ; in zoology, such notions as that a 

 Cephalopod is a vertebrate animal doubled upon itself, that an Insect 

 is a vertebrate animal with free ribs, &c. 



It is precisely on this footing however that at present our Common 

 Plan or Archetype of the Mollusca stands. We have before us the 

 evidence which might perhaps have satisfied Geoffrey and Oken. Given 

 our plan and certain laws of modification, and all known molluscau 

 forms may be derived from it ; but it remains to be seen how far tho 

 evidence which would alone have satisfied Von Biir, the evidence of 

 development, justifies the view which has been taken ; how far in fact 

 our hypothesis is capable of being elevated to the dignity of a theory. 



To this end it is by no means requisite to show that every Mollusc 

 has at one time the archetypal form, and is subsequently modified into 

 its persistent condition ; to maintain such a proposition it would be 

 necessary greatly to simplify (though not essentially to alter) tho 



