MOLLUSC A. 



365 



and complex, and in the Cephalopods is evi- 

 dently provided with gustatory papillae. The 

 sense of touch is in this as in other groups of 

 animals the most generally possessed and the 

 most widely diffused over the body. The mar- 

 ginal fringe of tentacles on the mantle of most 

 Bivalves ; the branched processes of the skin 

 in certain Pteropods and Gastropods ; the ce- 

 phalic tentacles or ' horns,' as they are termed, 

 in the Snail and allied Mollusks; the rich and 

 varied apparatus of cephalic and labial ten- 

 tacles nearly one hundred in number in the 

 Nautilus ; and the extremities of the acetabuli- 

 ferous arms of the higher Cephalopods, may 

 all be regarded as organs of delicate sensa- 

 tion : the same opinion may reasonably be 

 entertained of the highly vascular surface of 

 the ventral locomotive disc or ' foot' in the 

 Slug, Snail, and other Gastropods. 



The voluntary muscular fibre of the Mollus- 

 cous animals is distinguished from that of the 

 Articulate and Vertebrate animals by the ab- 

 sence of the transverse strise, and in most of 

 the Acephalous Mollusks it is antagonized by a 

 merely elastic gelatinous fibre. In the Tuni- 

 caries the flexible outer coat obeys and opposes 

 the change of form which the inner muscular 

 envelope occasions. In the Bivalves, whose 

 shell affords firm levers of attachment to the 

 muscles, the antagonizing elastic force is im- 

 planted at the hinge of the shell ; and some of 

 the species ( Mollusc a subsilentia of Poli) can, 

 by virtue of this mechanism of solid lever with 

 its attached muscular and elastic fibre, execute 

 short leaps. 



The Encephalous Mollusks with a cerebral 

 centre of nervous influence antagonize one 

 series of muscles by the regulated action of 

 another series, and are no longer dependent on 

 mere elastic fibres for their movements. The 

 musculo-cutaneous mantle is produced in the 

 form of fins in the Pteropods, Heteropods, and 

 some Cephalopods ; or there is an accumula- 

 tion of longitudinal fibres, intersected by ob- 

 lique or transverse ones on the ventral surface 

 of the mantle, producing the thick contractile 

 disc, which is termed the foot. This some- 

 times extends the whole length of the body, as 

 in the Gastropods, sometimes is developed only 

 from the region of the neck, as in the Trache- 

 lipods. The attachment of the body to the shell 

 in these Mollusks, the presence of, and power 

 of retracting and elongating, a siphon or breath- 

 ing tube, the movements of the head and its 

 appendages, especially when these are deve- 

 loped into instruments of progressive motion 

 and prehension, as in the Cephalopods, are the 

 chief conditions of the progressive advance- 

 ment and complication of the muscular system 

 in the Molluscous sub-kingdom. 



The heterogangliate type of the nervous sys- 

 tem, with the correspondingly low condition of 

 the muscular and other organs of relation, 

 which, commencing in the Ciliobrachiate Po- 

 lypes, is established in the Mollusks, is on the 

 whole very inferior to the conditions and powers 

 of the sensitive and motive systems in the Arti- 

 culates : but, on the other hand, " the organs 

 of growth and reproduction become more 



evolved ; and in the Mollusca we are presented 

 with a perfecting of the internal organs, which 

 is to prepare for and to be more fully deve- 

 loped in the higher animals."* 



The alimentary canal is provided with a se- 

 parate mouth and vent : the stomach may be 

 distinguished from the oesophagus and intes- 

 tine, and a liver is present in all the Mollusks, 

 and is remarkable in most for its large size and 

 complicated lobular form. The bile is secreted 

 from arterial blood. The mouth in the Ace- 

 phalous Mollusks is generally concealed in the 

 interior of the pallial cavity, but is always si- 

 tuated at the anterior part of the body, or that 

 which is opposite to the excrementory and re- 

 spiratory tubes or orifices. There are neither 

 jaws nor salivary glands in this division. Among 

 the Encephala, however, in which the mouth 

 is armed with horny plates representing in diffe- 

 rent species the knife, the saw, the rasp, or the 

 scissors, the salivary system attains an extraordi- 

 nary degree of development, especially in the 

 Cephalopods, in which the mandibles resemble 

 those of the Raptorial bird, and sometimes have 

 their margins armed with a calcareous dentated 

 plate. In the Cephalopods the pancreas makes 

 its first appearance. The special forms of the 

 progressive complications of the digestive sys- 

 tem in the present division of animals will be 

 found amply illustated in the articles TUNI- 

 CATA, CONCHI.FERA, PTEKOPODA, GASTRO- 

 PODA, CEPHALOPODA. 



The circulating system, which, as has been 

 stated, is complete and double in all the 

 Mollusks, is provided in all the species with 

 a systemic heart, and in the highest organized 

 species with a distinct heart for the lesser or 

 branchial circulation. The systemic heart first 

 appears in the sessile Tunicaries as a vasiform 

 undivided ventricle, which, however, is in- 

 closed in a distinct pericardium. It is concen- 

 trated into a more compact muscular organ in 

 the Conchifers, and divided into a venous and 

 arterial chamber; but the auricle, and some- 

 times also the ventricle, as in Area, is sub- 

 divided and segregated, according to the law 

 of self-repetition, which is exemplified in all 

 the systems of organs at their first appearance 

 in the animal kingdom. The heart of the 

 Gastropods exhibits the higher type of a single 

 auricle and ventricle, both placed at the termi- 

 nation of the lesser and the commencement of 

 the greater circulation. In the highest Cepha- 

 lopods the course of the blood is accelerated 

 in both circulations by a muscular ventricle, 

 but the superadded analogue of the pulmonic 

 heart here, likewise, at its first appearance 

 illustrates, by its separation into two distinct 

 ventricles, the law above alluded to in reference 

 to the systemic heart of the lower Mollusks. 



The respiratory organ is distinctly developed 

 in all Mollusks, and is subject to the greatest 

 variety of forms in this division of the animal 

 kingdom ; yet, with the exception of the small 

 sessile order of Ascidians at the lowest extreme, 



* See the eloquent and philosophic ' Recapitu- 

 lary Lecture" of Prof. Green, Vital Dynamics, 8vo. 

 1840. 



