' 



MOLLUSCA. 



MOLLUSCA. 



of the integument In the last two there is such a permanent 

 inversion. 



In the Cyciatlomala the horny or calcareous deposit in the iutegu- 

 meat of the abdomen joins the soft parts by an even level edge, and 

 there is nothing which serves as a cover or operculum, for the retracted 

 Polygon. 



In the Ctcaottomata (fg. 2, 2) the margins of that portion of the 

 abdomen which in inverted in the retracted state are produced into 

 a toothed horny sheath, which can be retracted by special muscles, 

 and which serves as an operculum. 



In the Chrtluitomala (Jig. 2, 1) the horny or calcareous deposit takes 

 place in such a manner that the hardened integuments ^of the front 

 portion of the eternal region constitutes a sort of lid, regularly articu- 

 lated upon the hinder portion, and provided with proper occlusor (and 

 perhaps levator f) muscles. It should be noted that the anal aperture 

 la directed away from this lid or operculum. 



In each of the previous divisions the tentacles are arranged on a 

 circular disc, or lophophore, of whose edges they are prolongations ; 

 but in the great majority of the Jfippocrtpia (Jig. 2, 3), which are all 

 fresh-water forms, the lophophore is so produced into two arms on the 

 anal side as to assume a horse-shoe shape. It is important to con- 

 sider this in connection with the peculiar features presented by the 

 Bratkiopoda, 



Thirdly, we venture to regard the peculiar genus Paliccllina (Jig. 2, 4) 

 as constituting on order by itself. Essentially a Polyzoon, it is neverthe- 

 less distinguished from all other Polyzoa by the circumstance that its 

 tentacles are united together by a membrane into a cup, which cup is 

 never protruded far beyond the general boundary of the body. 



The CKMnttomata arc remarkable for possessing two kinds of moveable 

 appendages Flabeliaria, whip-like processes, articulated to a bulb 

 containing muscles by which they are moved ; imdAricularia or bird' s- 

 head processes (Jig. 2, 5). The structure of the latter is of great interest 

 in a morphological point of view, and demands particular attention. 

 They consist of a larger piece, or valve (;>), shaped like a bird's head 

 and produced into a longer or shorter process of attachment, to which a 

 smaller valve (o), representing the bird's lower jaw, is articulated. 

 Stalked or sessile, these avicularia present during life an incessant 

 mapping action, produced by the alternate contraction of two seta of 

 muscles, which arise from the concavity of the ' skull ' of the bird's 

 head by wide fan-shaped origins, aud seem to be inserted by narrow 

 tendons into the smaller articulated valve. The one tendon (<) is inserted 

 into the smaller valve in front of the line of articulation, and the 

 other {) behind it, and therefore by their alternate action they raise 

 and depress the lesser valve upon the larger. 



Fig. S. 



friltacta. 



a, oral aperture ; , anil tpcrturr, or extremity of the intwtinc ; I, adductor 

 sales of BrmtUefoda ; n, cardinal muscle* of llrachiupoda ; p, pedicle ; 

 / r -, pcdkl* mujclw ; f, pedal ftaglion. 



The Brackiopoda. Now, if we compare the relative positions and 

 mode of articulation of the operculum and cell of a Chcilostomatous 

 Polyzoon, or of the two valves of an avicularium, with those which 

 obtain in the nhtlU of the typical Brachiopoda, such as the Ttrtlra- 

 ItUitla and flfiytidumdlidcr. the resemblance will be found to be very 

 striking ; and still more so, if in addition the arrangement of the muscles 

 be taken into consideration. In such a Brachiopod, in fact (Jig. S), the 

 bell is composed of two valves one large, excavated, and produced into 

 a canal or tube, through which a pedicle of attachment passes ; while 

 the other is smaller and more or leas flattened. The two valves are 

 articulated together by means of a socket in the smaller valve and a 

 tooth in the larger, on each side, the intermediate space being free, 

 iust as the operculum'of the Polyzoon is united with its cell, or as the 

 leaser valve of an avicularium is articulated with the larger. So 

 Ukewue the anal extremity of the Brachiopod is turned from the 

 mailer valve. Then the arms of the Brachiopod are essentially com- 

 parable to those of the lophophore of a Hippocnpian Polyzooa, 

 except that their direction is different; the calcined rapports to which 

 they are fixed in taany Brachiopoda, are so variable in form and so 

 extensively absent in others, that their existence can in nowise affect 



the homology of the parts. Again, if we leave out of consideration 

 the pedicle-muscles (which are however, in all probability, as Mr. Han- 

 cock as shown, the homologues of the retractors of the Poltpoa), the 

 arrangement of the other muscle* is precisely what wo have seen to 

 obtain in the nvicularium : the adductors which pass from the larger 

 valve to be inserted into the smaller, in front of its point of support, 

 corresponding precisely with the occlusor muscles of the avicularium ; 

 while the cardinal muscles, which arise from the larger valve, and past 

 to be inserted into the cardinal process of the smaller, behind the 

 point of support, are identical with the divaricator muscles of the 

 avicularium. 



The existence of distinct muscles for the purpose of separating the 

 valves of the shell is characteristic of the 1'olytua and Brachiopoda, 

 the only approximation to such an arrangement at present known 

 among the Lamcllibranchiata being presented by the Pholada. 



Finally, if the great proportional size of the Brachiopoda, their 

 pedunculated attachment, their thick and solid shells, and their 

 simple forms, be brought forward as arguments against the view we 

 take of their essentially polyzoic nature, we would remind the objector 

 of the like opposition in such features between Soltenia and JiotryUiu, 

 or Aplidium, among the Ascidians. 



Two principal modifications of the common Brachiopod plan are to 

 be observed. In the Tercbratulid<K and JUiynchonettida, and in all 

 probability in their extinct allies the Spirifcrida, Orthidd, and Pro- 

 ductiiltt, the muscles are always arranged in three sets Adductor, 

 Cardinal, and Peduncular. At the same time the mantle (whose 

 homology with the produced edges of the non-retractile port of tho 

 abdomen of a Polyzoon is at once appreciable), though divided into 

 two distinct lobes in front, is continuous and entire behind, that in, 

 towards the peduncle. A still more remarkable feature in their 

 organisation is that, at least in Waldlieimia and RhynchoneUa, there is 

 no anal aperture, the intestine terminating in a ccocurn directed 

 towards the middle of the large valve. 



In the Craniada, Ditcinida, and Linyulid<e the muscles have a very 

 different arrangement, which could only be rendered intelligible by 

 detailed descriptions and illustrations, as the homologies of these 

 muscles with those of the other division are not yet determined. 

 The lobes of the mantle again are completely separated (Ditcina, 

 Linyula, Crania ?), and the intestine opens upon one side of the body 

 between these lobes. There are no teeth, and the articulation of one 

 valve with the other and the modes of attachment vary remarkably; 

 Linywla having a long peduncle ; Croatia being attached by the surface 

 of its lower valve; and/>ucina having an aperture in the correspond- 

 ing valve through which a portion of the adductor passes, aud 

 spreading out at its extremity into a sort of plug, acts as a pedicle. 



Neural MuUtuca. The La.mcllAranckia.ta. In all JUolliaca proper 

 the neural region is developed to a much greater extent than in 

 the Mottuicoida, and there are always three pairs of ganglia, two 

 Cerebral, two Pedal, and two Parieto-Splanchnic (or branchial). Tho 

 especial characters of the Lauulliuranchiata, as modifications of the 

 Archetype, are the following : The haemal region is well developed 

 in its abdominal portion, but forms no prominent sac-like abdomen, 

 into which the viscera enter in the adult condition. Ita edges aru 

 produced into extensive pallia! lobes, which are arranged on each side 

 of a longitudinal plane, and not above and below a horizontal one 

 (or more properly before and behind a transverse one), as in Hi 

 Brachiopoda. The mouth is surrounded by a fringe, representing 

 the tentacles in the Moliutcoida (as may be well seen in I'tctai, Jig. 4,4) 

 which is produced laterally into elongated ' palps,' but is totally unpro- 

 vided with any manducatory apparatus. The intestine passing from tho 

 stomach either forms a simple loop with a second open angle directed 

 hicmally, or this loop may be much coiled aud convoluted : tin: 

 intestine finally passing over the great posterior adductor and termi- 

 nating between the lobes of the mantle behind it 



The foot may bo more or less largely developed, but never presents 

 any clear distinction into pro- meso- and meta-podium, unless indeed, 

 as we are inclined to suspect, the whole free portion of tho foot of the 

 LamcUibranckiata ought to be regarded as a modified metapodiuui. 

 Besides the pedal muscles, the Lamellibrauchs possess one or two charac- 

 teristic muscles the adductors, which approximate the valves of the 

 shell, and whose greater or less development seriously aflects tho 

 ultimate form of the animal 



The gills deviate but little from their archetypal form and position 

 in some Lamelu'bnuichs, such as Triyonia and Pectcti, being merely 

 thrown downwards by the development of tho mantle. In A'. c/a 

 (Jig. 4, 8), their inner edges are united posteriorly, but they remain com- 

 paratively small lu the majority of Lamellibrauchs however, tho 

 gills are exceedingly large in proportion to the real of the body, and 

 consist of two double plates, which are united with the mantle and 

 with one another, in such a manner as to divide the pallia! cavity into 

 two chambers, a supra- and infra-branchial, which communicate only 

 by the passage between the anterior edge of the bronchia and tliu 

 foot, and by the multitudinous perforations in the branchial plates 

 themselves. 



It is in the absence of external organs of sense or of any buccal mas- 

 ticatory apparatus, and in the peculiar arrangement of the gills, that 

 the main difference between the LamMibranchiata and the 0'iutero- 

 lies; and hence the great resemblance which the ideal section of 



