865 



MOLLUSCA. 



MOLLUSCA. 



8C8 



branchial sac, which 13 proportionally small, and these might therefore 

 be termed Asctdice Intestinales, Intestinal Ascidians. A very complete 

 mutual representation will be found to obtain between the members 

 of these two groups. 



Hmnal Mollusca. In passing from the H;cmal Molluscoida to the 

 Hoemal Mollusca, we find the same new features presenting themselves 

 as in the Neural Division, the transition being even more abrupt, from 

 the absence of any representative of the Lamdlibranchiata. In all 

 these Mollusca, in fact, there is a more or less well-developed foot ; 

 a distinct head, with its organs of sense and buccal armature ; and 

 three pairs of ganglia cerebral, pedal, and parieto-splanchnic. 



The modification of the Common Plan is carried to a less extent in 

 this than in the Neural Division, the chief varieties of its forms 

 depending on the changes in the shape of the shell with which the 

 majority are provided; on the greater or less development of the 

 different regions of the foot ; but most of all in the relative pro- 

 portions of the mesosoma and mantle. 



If we divide the Haemal MoUueca, into two great groups the one 

 consisting of the Hettropoda, Scutibranchiata, Tubulibranchiata, Pectini- 

 brandtiata, and Cydobranchiata, families which are most intimately 

 allied, and which are connected as a group by the dicecious arrange- 

 ment of their reproductive organs ; and the other of the Nudibran- 

 chiata, f aferol/ranchiata, and Tectibranchiata, families in like manner 

 united, among other characters, by their common hermaplirodism, 

 then we shall find in each such group two extremes of form the 

 one resulting from the great development of the pallial region, the 

 other from that of the mesosoma. In the Dicccious Division, Dentalium, 

 Ytrmetut, Atlanta, and the ordinary 1'ectinibranchiata may be regarded 

 as examples of the former case ; and in the Moncccious Division the 

 Inferobranchiata and Tectibranchiata ; while the mantle becomes 

 rudimentary or absent altogether in the Dicocious Firoloides, in the 

 Monoecious Phyllirhoe, and the Nv.dibranch.iata in general, where the 

 region from which the so-called branchial processes arise, and which 

 i commonly called the mantle, is not the homologue of the mantle of 

 Atlanta for example, but of its mesosoma, which here, as in Firoloidei, 

 constitutes the main portion of the body. 



Fig. 8. 



Hcteropoda. 8, Atlanta ; 9, Firohid^t. 



a, -oral aperture; b, anal aperture, or the extremity of the intestine; mt, 

 mantle ; mt, mcsopodium ; pp, propodium ; r, ventricle. 



The foot in the Moncccious Eternal Molluica rarely presents any 

 special development of its different regions, except that in certain 

 forms namely, Aplysia and Gasteropteron the epipodium is as well 

 marked an in the Pteropoda, and serves the same end in locomotion. 

 This is well known in Gaiteropteron, and we have seen a tropical 

 Aplyria 'By' through the water in precisely the same way as a 

 Pteropod would do. These epipodial lobes have been frequently 

 called mantle, although the true mantle ia a most distinct and obvious 

 tructurt. 



In the Dicccious group the epipodium is never well developed, 

 presenting itself at most under the form of little lobes and processes 

 at least it would seem probable that the neck-lappets and head- 

 lappeU of tho Trochidce are rudiments of the epipodium. On the 



HAT. HI8T. D1V. VOL. III. 



other hand, it is in this group that the propodium, mtsopodium, and 

 metapodium attain their most complete and distinct form ; as in 

 Atlanta, where the propodium constitutes the anterior flattened fin, 

 the mesopodium the rounded sucking disc, and the metapodium 

 extends backwards, as the tail-like lobe which carries the operculum. 

 In Firoloides we find that the mesopodium has vanished, and the 

 metapodium has taken the form of a mere filament, while the pro- 

 podium constitutes the great swimming fin. 



Fig. 9. 



Foot of Pectinibranchiata. 1, Trochtts ; 2, 3, Nutica. 

 a, oral aperture; mt, metapodium ; mt, mesopodium; pp, propodium. 



In the ordinary Pectinibranchiata, on the other hand, the foot may 

 not be differentiated into its subdivisions at all, the metapodium being 

 marked only by the position of the operculum, when this exists, as in 

 Buccinum. In other cases, as in Oliva and Sigaretus, a deep cleft 

 marks off a very distinct propodium from the conjoined mesopodium 

 and metapodium ; in others, as in Pteroceras, the metapodium is as 

 specialised as in Atlanta; while again, in such forms as Natica, the 

 three constituent parts are distinguishable the propodium consti- 

 tuting the hood in front of the head ; the mesopodium the creeping 

 disc ; and the metapodium the operculigerous lobe. (Fig. 9, 2 and 3.) 



Having thus passed in review those modes of arrangement of the 

 various organs of the Mollusca which constitute the Common Plan of 

 the group and the subordinate plans of its leading subdivisions, we 

 have next to consider the peculiarities presented by these organs 

 themselves, or, in other words, those more striking features in which 

 the organs of the Mollusca differ from those of the Vertebrala, Annu- 

 loia, and Radiata. The most important organs, in this point of view, 

 are those of 1, the Alimentary ; 2, the Circulatory ; 3, the Kespira- 

 tory ; 4, the Renal ; and 5, the Nervous System.* 



1. The Alimentary Organs, in certain Mollusca, present two kinds 

 of apparatus which are met with in no other division of the Animal 

 Kingdom. The first of these is that peculiar manducatory instrument 

 usually called the ' tongue,' which is possessed by all the Mollusca, 

 proper, except the Lamellibranchiata ; and for the first description of 

 whose true structure and mode of action we are, we believe, indebted to 

 Mr. Thompson (see article ' Tongue,' in the ' Cyclopaedia of Anatomy 

 and Physiology '), although the organ itself had been more or less au 

 object of attention ever since the time of Cuvier. 



Fig. 10. 



Tongue of Patella. 



1. aa, the cartilaginous plates which constitute the pulley over which the 

 elastic plate 2, b t supporting the series of teeth c, plays ; d and e are the 

 anterior and posterior insertions of the intrinsic muscles of the tongue. 3 ifl 

 a side view, and 4 a view from above, of the entire apparatus. 



The tongue is essentially composed of a cartilaginous mass, with a 

 pulley-shaped upper and anterior surface, which projects from the 

 bottom of the oral cavity. An elastic plate plays over the pulley, and 

 ia attached at each end to muscles which arise from the upper and 



* Our limits preclude the consideration of the tcgumentary and genital 

 systems, whose peculiarities however are less exclusively Molluacan. 



3 K 



