857 



MOLLUSCA. 



MOLLUSCA. 



S58 



This figure is supposed to be bilaterally symmetrical, and the 

 following parts and regions are to be distinguished in it : (If). The 

 Haemal Region, or that upon which the heart is situated, and which 

 corresponds with what is commonly termed the dorsal region. The 

 word dorsal, however, is vague, being used in different senses in 

 various divisions of the animal kingdom, and should therefore be, 

 abandoned in philosophical anatomy. For the same reason, the 

 opposite region (A') is termed, not ventral, but Neural, inasmuch as 

 it is the region in which the great centres of the nervous system are 

 placed. The termination (a) is the anterior or oral; the end (b), 

 the posterior, or anal. Between these extremities the intestine takes 

 a straight course. The neural surface is that upon which the 

 majority of Molluscs move and by which they are supported ; and it 

 is commonly modified to subserve these purposes into a muscular 

 expansion or disc called the Foot Three regions again, often very 

 distinctly divided from one another, may be distinguished in this foot : 

 an anterior, the Propodium (pp) ; a middle, the Mesopodium (ms) ; and 

 a posterior, the Metapodium (mt). In addition to these, the upper 

 part of the foot or middle portion of the body may be prolonged into 

 a muscular enlargement on each side, just below the junction of the 

 haemal with the neural region the Epipodium (ep). The mass of the 

 body between the foot proper and the abdomen, or post-abdomen, which 

 bears the Epipodium, and whose limits cannot very well be denned, 

 though it would be very convenient to have a name for it, may be 

 termed the Mesosoma (mid-body) ; and for what is loosely called the 

 head the name Prosoma might advantageously be adopted. On the 

 upper part of the sides of the head or Prosoma are two pairs of 

 organs of sense : the Eyes (which may be supported on pedicles 

 Ommatophores), and the Tentacles. In the haemal region the integu- 

 ment may be peculiarly modified and raised up at its edges into a 

 free fold, either in front of or behind the anus, and when so modified 

 it is called a Mantle (Pallium). In front of the anus again the 

 Branchiae () project as processes of the haemal region. Among the 

 internal organs we need only point out the position of the Heart (u t v) t 

 which lies in front of the branchiae in the haemal region ; and the 

 Nervous Ganglia (x, y, 2), of which there are three principal pairs 

 arranged around the alimentary canal, which they encircle by means 

 of their commissures. 



Such is the Common Plan of which all Molluscs whatsoever may 

 be regarded as modifications ; the next question is, to consider the 

 laws according to which the plans of the great sub-classes of the 

 ifollutca may be derived from it. 



2. Modification* of the Common Plan. The structural peculiarities 

 of all known Molluscs may be very simply accounted for by the 

 excessive or defective relative development of certain regions in the 

 Archetype, more particularly of one or other parts of the Haemal 

 Region. Of this region the portion which lies in front of the anus may 

 be conveniently termed the Abdomen, while to that which lies behind 

 it the term Post-Abdomen may be applied. Now, if it be supposed 

 that the Abdomen grows out of proportion to the rest of the body, 

 constituting a kind of prominence, and that the intestine passes into 

 the outgrowth so as to form a sort of loop (n.), it is clear that the 

 open angle of this loop will be turned towards the Neural surface ; 

 and the intestine may be appropriately said to have a Neural flexure. 

 On the other hand, if it be supposed that the Post-Abdomen grow 

 out in the same way, and draws into itself a loop of the intestine, 

 then the open angle of the loop will be in the opposite direction, thai 

 is, it will be directed towards the Haemal surface ; the intestine there- 

 fore may in this case be said to have a Haemal flexure (in.). It will be 

 readily understood that either Abdomen or Post-Abdomen may 

 develop a mantle or not, and that the existence or absence of this 

 mantle has nothing to do with the essence of the change in question 

 however much it may affect the external appearance of the resulting 

 form. 



Again, the extent to which the Abdomen or Post-Abdomen is 

 developed, may have a great influence on the relative position o 

 certain organs of the Mollusc. Thus, in the first place, the position 

 of the anus may become greatly altered. When there is a neural 

 flexure it will acquire a direction towards the neural surface and 

 backwards, the final approximation to the oral end depending on the 

 amount of the development of the abdomen on the one hand, and 

 that of the neural region on the other. Again, if the outgrowth of 

 the abdomen take place, not symmetrically, but more or less on one 

 side of the median line, the final position of the anus will be towards 

 the opposite side and to the right or left, as the case may be. 



It is even conceivable (this amount of modification indeed actually 

 obtains in nature) that by an exceedingly one-sided development of the 

 abdomen, the anus may be thrust quite round on to the haemal side. 

 Its final position therefore must not be regarded as certainly indica- 

 tive of the direction of the flexure by which it obtained this position. 

 Where there is a haomal flexure again, the direction of the anus will 

 bo normally towards the haemal (that is, dorsal) side, and forwards; its 

 npproximation to the head, its asymmetrical position, and the amount 

 to which it may be thrust backwards and towards the neural side, 

 depending upon conditions of the same order. 



It is not merely the anus which is affected by these changes however; 

 the branchia; (and the heart which follows them) undergo similar trans- 

 positions, whose nature and origin it is very necessary to understand 



n order to appreciate their value as organic characters. M. Milne- 

 Edwards long since pointed out the singular fact that, in certain 

 Molluscs, the branchiae are in front of the heart, while in others they 

 .re behind it. The latter he termed Opisthobranchiala, the former, 

 Prosobranchiata. It will be seen that our Archetype is Opistho- 

 jranchiate. Now, it is easy to understand that if an Abdomen were 

 developed in front of the heart, without involving the cardiac region, 

 the Mollusc would remain opisthobranchiate ; if however it were 

 more extensively developed so as to involve the heart and branchiae, 

 ,he heart, from having been in front, would eventually take a position 

 josterior to the branchiae, and the Mollusc would thus become proso- 

 Dranchiate. So with regard to the development of a Post-Abdomen ; 

 its effect on the position of the heart and branchiae would depend 

 wholly on the extent of haemal surface which it involved. It follows, 

 therefore, that Opisthobrauchism may co-exist with either a haemal or 

 a neural flexure, or with none ; while Prosobranchism indicates one or 

 the other, but not which ; and that these organic characters, however 

 valuable, are secondary to and therefore of less importance than the 

 neural and haemal flexurea (that is, development of an abdomen or 

 post-abdomen), on which they depend. Dealing with the facts fur- 

 nished by adult structure alone then, there are two primary modifica- 

 tions of the Molluscan Archetype, which may be shortly termed tlie 

 Neural and Haemal Plans. The Cephalopoda, Pulmonata, Pteropoda, 

 Lamellibranchiala, Srachiopoda, and Polyzoa, are the molluscs 

 which present madifications of the Neural Plan. The Heteropoda, 

 Gasteropoda, Tectibranchiata, Inferobranchiata, Cyclobranchiata, Tubuli- 

 branchiata, Nudibranchiata, and Aicidioida, are those which present 

 modifications of the Haemal Plan. 



3. The Neural Plan and its Principal Modifications. Milne-Edwards 

 has proposed a division of the Mollusca into the Mollusca proper, and 

 the Molluscoida (Molluscoidcs), including under the latter class those 

 Polype-like forms, the Polyzoa and the Ascidioida. Believing that 

 the Molluscoida, are as truly and wholly Molluscau as any other 

 Mollusca, we nevertheless consider the distinction drawn by the eminent 

 French naturalist to be very important, and that it should be retained 

 as a primary subdivision of the great Haemal and Neural Divisions. 

 In the haemal division the limits of the Molluscoida are the same for 

 us, as for M. Milne-Edwards ; but in the neural we include somewhat 

 more. In fact, if the most fitting definition for this subdivision be 

 those Molluscs which have the neural region comparatively little 

 developed and the nervous system reduced to a single or at the most 

 a pair of ganglia, while the mouth is usually surrounded by a more or 

 less modified circlet of tentacles, then we shall find that, in the neural 

 division, we must include the Srachiopoda with the Polyzoa. Commenc- 

 ing our study of the morphology of the special groups of the Mollusca 

 with the Neural Division ; and with the Molluscoid sub-division of 

 the neural forms then, we have to consider first, the Polyzoa and the 

 Srachiopoda : 



Fig. 2. 



. 1, Membranipom. 2, Boiecrliankia. 3, riumatella. 4, 2'ediccllina. 



5, Avicttlarium. 



The Polyzoa. Conceive the abdomen of the Archetype to be greatly 

 prolonged, the neural region with its appendages, the organs of sense, 

 and the heart, remaining undeveloped ; so that the anus comes into 

 close apposition with the oral extremity, while the edges of the latter 

 are produced into long ciliated tentacles, and the result will be a 

 Polyzoon, which needs only the power of gemmation to give rise to 

 those composite aggregations which are so characteristic of the 

 group. 



The Polyzoic type itself presents five subordinate modifications m 

 the five principal orders of the group -.the Cyclostomata, Cteiw- 

 stomata, Cheilostomata, Ilippocrcpia, and Pcdicillinida. 



In the first three, the body of the Polyzoon when fully expanded is 

 completely straightened, there being no permanent fold or inversion 



