681 



MOLLUSCA. 



MOLLUSCA. 



LamfUilr ant-hint a. 1, Lutraria. 2, Vnio. 3, Nucula. 4, Ptcten. 



a, oral aperture ; i, anal aperture, or extremity of the intestine ; c, renal 

 organ ; w, mantle ; r, labial palpi ; * *', anal and branchial siphons ; f, 

 branchiic ; r, ventricle; y, pedal ganglion; A, anterior adductor; If, posterior 

 adductor. 



a typical Lamellibranch bears to a typical Gasteropod. Compare 

 (fig. 4) 4 with with 1, 3, and 2. 



It may seem at first sight inconsistent with our own principles to 

 consider as neural molluscs these Lamellibranchs, which confessedly 

 have flle principal loop iu the intestine open to the h:einal side. But the 

 position of the largely-developed mantle, completely in front of the anal 

 aperture, and the direction of the aortic end of the heart, unchanged 

 from what is observable in the Archetype, are sufficient, apart from 

 developmental considerations, which will be adduced by aud bye, to 

 prove that the second flexure of the intestine in this case is to be con- 

 sidered accidental, the result of the great development of the meso- 

 soma, to serve as a chamber for the viscera, and of the enlargement 

 of the great posterior adductor, thmsting up the rectum which 

 passes over it. 



As for the leading varieties of form of the Lamellibranchs, there 

 are none which, in reality, depart very widely from the Common Plan. 

 Perhaps Teredo or Pholai, on the one hand, and Ostrcca, on the other, 

 may be regarded as the extreme forms, the former being as much as 

 possible elongated longitudinally, the latter attaining the extreme of 

 concentration about a centre. At the same time there is a reduction 

 of parts to a minimum, as shown in the absence of a second adductor, 

 and of any foot in the adult state. The differences between these 

 forms are however decidedly less than those which may be observed 

 between the extreme forms among the Ctphalopoda or Gasteropoda. 



The Pteropoda and Pulmonata. The Lamellibranchs are, as we 

 have said, curiously exceptional in presenting the general features of 

 the Mollusca proper, without that singular buccal apparatus which we 

 meet with in all other members of the subdivision, whether neural 

 or htemal, and whose peculiar nature is described below. Again, they 

 are exceptional in the vast development and symmetrical longitudinal 

 division of their mantle, and in the corresponding division of their 

 pallial shell into two pieces or valves characters we shall not meet 

 with again in any modification of the Common Plan. 



In the Pteropoda and Pulmonata the mantle is never developed into 

 such lateral lobes, and the shell to which it gives rise never consists 

 of two pieces, but is constituted by a single mass, which either has 

 the form of a flat plate or presents some modification of a cone. 

 Again, the foot (or some part of it) is always well developed, present- 

 ing no obvious distinction into regions in the Pulmonata ; but in the 

 Pteropoda often exhibiting a well-marked meso- and meta-podium, aud 

 always presenting a characteristically large epipodium an organ which 

 iu these Molluscs constitutes the so-called ' wings," from which their 

 name is derived. 



Fig. 5. 



Pteropoda. 1, Pneumodermon. 2, Cleodora. 3, Psyche (foot and head only). 

 Letters as in figure 1 . 



There is usually a well-developed mantle iu the Pteropoda and 

 Pulmonata, and its walls act as a branchial surface without being 

 produced into true gills (Hyalcea ?) the sea-water in the marine 

 Pteropoda and the air in the terrestrial aud aquatic Pulmonata being 

 inspired and expired into its cavity. 



In the Pteropoda in general, the aperture of the pallial cavity and 

 that of the anus, are situated upon the posterior surface of the body, 

 in accordance with the neural flexure of the intestine. The anal 

 aperture however is usually thrust to one side of this surface, and, iu 

 Limacina and Spirialis, this lateral thrust has taken place to such an 

 extent, that not only the anal aperture, but that of the mantle cavity, 

 is thrown up completely on to the dorsal surface. This latero-dorsal , 

 or dorsal position of the anal and respiratory apertures, is as regular 

 in the Pulmonata as it is exceptional in the Pteropoda. 



In the Pteropoda and Pulmonata some most important modifications 

 of form are produced by the greater or less development of the meso- 

 soma on the one hand and of the mantle on the other. The predomi- 

 nance of the latter is to be observed in such forms as Oriseis, Cleodora, 

 Hyalcea, and Heli:c ; while the former may be seen in Pneumodermon 

 and in Limax. In the latter the mautle is very small, and in the 

 former it is almost if not entirely absent ; what is ordinarily considered 

 as the mantle in this mollusc being in fact nothing more than the 

 mesosoma. The like confounding together of parts so essentially 

 different has taken place, we shall find, in the Nudibranchiata, and in 

 the lleteropoda. 



The Cephalopoda. In the Pteropod forms, Pneumodermon aud Olio, 

 a hood, giving off long processes covered with suckers from its inner 

 surface, surrounds the oral aperture, and there is every reason to 

 believe corresponds with the propodium, whose lateral halves have 

 united over the mouth. If the like process were to take place in a 

 Criecis, but to a greater extent, so that the mouth were thrust back 

 between the halves of the mesopodium, and the propodium aud 

 mesopodium formed one continuous tentaculigerous sheath around 

 the oral aperture ; and if at the same time the two halves of the 

 epipodium united posteriorly into a funnel-shaped tube, the Griseis, 

 BO far as its external organisation goes, would no longer be a Pteropod, 

 but would have become a Cephalopod. In fact, the Cephalopod may 

 be derived from the Archetype by supposing these modifications. The 

 mantle ia always well developed, and its cavity incloses one or two 

 pair of gills. The two halves of the epipodium are united behind into 

 what is called the funnel, a peculiar apparatus, of great importance 

 in the economy of many Cephalopoda ; and in the majority of the 

 group the sides of the foot, having united in front of, and forming a 



