M7 



MOLLUSCA. 



MOLLUSCA. 



lower surface* of the cartilaginous mass. Along the middle Una of 

 thi* elastic plate snooestiv* transverse serial of strong recurred teeth 

 re set new one* being continually formed behind u the old are worn 

 way in tort of persistent dental sac. 



When the tongue is brought into play it is protruded by appro- 

 priate muscles from the cavity of the mouth, and it extremity ia firmly 

 applied against the body to be rasped. The superior and inferior nets 

 of muscles, which are inserted into the corresponding ends of the 

 elastic plate, now contract alternately, and the resulting action is pre- 

 cisely that of a circular saw. It U by means of this apparatus that 

 the Carnivorous if oil tufa bore through the shells of the animals upon 

 which they prey ; and perforated sheila which hare been thus emptied 

 abound on every coast. 



The other appendage of the alimentary canal peculiar, so far as we 

 at present know, to the Molliuca, is what is termed the Crystalline 

 Style, transparent, usually elongated body, which projects by one 

 cud into the stomach, and is lodged for the rest of its extent in a 

 sac formed by a diverticulum of that organ. The Crystalline Style 

 is found in a great number of Lamellibroncbs (to which group it has 

 erroneously been supposed to be confined), but has hitherto been 

 observed in only a few Pectinibranch Molliuca, such as Plerocenu, 

 Slrombta, Trochut, and Afarex. Its function is wholly unknown. 



Among the alimentary appendages, the Liver in one group, the 

 Ascidisng, departs sufficiently from the ordinary plan to deserve par- 

 ticular notice. In these animals (fig. 1, No. 4, t) it always consists of a 

 series of narrower or wider anastomosing tubules, commencing in casca 

 upon the outer surface of the intestine, which they envelope in a close 

 network, and terminating by a narrow duct in the stomach. In the 

 Jiotryllulir the hepatic tubules are remarkably wide. 



'i. The nature of the Circulatory System in the Afollutca is at pre- 

 sent in some respects a vexed question, more especially as regards the 

 important point whether they possess a true closed system of vessels 

 or not. Without entering into any discussion of tho various arguments 

 used on both sides of a dispute which is in some respects verbal, we 

 may be permitted shortly to state our own conclusions on the 

 subject 



In the Polytoa there ore no special circulating organs, if we except 

 the cilia with which the perivbceral cavity is often lined, and which 

 keep up a continual current in the perivisceral fluid ; nor do we 

 imagine that any one will insist that in them the perivisceral cavity is 

 not a sinus, but has a truly venous lining membrane. 



In the Aicidiaiu there is a heart, but it is a simple muscular sac, 

 open at each end, and possessing the extraordinary power of reversing 

 the direction of its contractions, and thus circulating its blood first in 

 one way and then in the opposite. The blood thus poured out ia 

 driven through channels in which assuredly no separate lining mem- 

 brane is demonstrable. Indeed it is difficult to comprehend how any 

 one with a living Ascidiau under his microscope can question that 

 here, at any rate, the circulation takes place through lacuna-, and not 

 through vessels with distinct wall*. 



In the Brachiopoda a very remarkable vascular system has been said 

 to exist, consisting of two hearts (in RlimchoncUa of four), each com- 

 posed of an auricle and a ventricle ; the former being in free com- 

 munication with the perivisceral venous sinuses (perivisceral cavity, 

 nobit), while the latter ends in on aorta, whose branches undergo a 

 regular distribution. Such is the circulatory system in the llrachio- 

 poda according to Professor Owen ; but our own inquiries have tended 

 to strengthen very greatly the doubts first raised by Mr. Hancock as 

 to the true nature of this so-called circulatory system. In fact these 

 inquiries lead us to doubt whether the so-called 'hearts' of the 

 Brachiopoda have anything at all to do with the circulating system ; 

 Inasmuch as, in the first place, we are pretty confident that no ' arteries' 

 are given off from the apices of the ' ventricles,' as has been said, and 

 think it more than probable that they open externally. Secondly, 

 there is no evidence at present, either indirectly from structure or 

 directly from observation during life, that the so-called ' hearts' of 

 any Urachiopod are contractile. Thirdly, the multiplication of these 

 hearts to four in flhynchonclla seems not a little to militate against 

 their cardiac nature. 



We may fairly conclude then that, for the present, the nature of the 

 circulatory system in the BtmtMofoda must be regarded as an open 

 question. 



Molltuea Proper. Tbo doctrine first advocated by M. Milne-Edwards 

 that in thete Molluscs the circulating system is always more or leas 

 incomplete, ban met with a wide acceptance, but also with no small 

 opposition. So far as the minute transparent Molluscs, which can be 

 submitted to direct microscopical observation during life, are concerned, 

 we do not understand how the truth of M. Milne-Edwards's doctrine 

 can be questioned. If the term 'venous lining* is to have any 

 meaning but a non-natural one, assuredly it cannot be said with truth 

 that anything of the kind exist* in the sinuses of Piroloidei, or of 

 Atlanta, or in those of the Plcropoda. 



In the larger Multutcn, on the other hand, much depends on the 

 verbal question what is the definition of a ' vein,' or ' venous mem- 

 brane f ' If a lamina of connective tissue separable from the surround- 

 ing part* be a venous wall, then doubtless the venous blood-channels 

 of many I>anitllibrancbs and Gasteropoda, and perhaps of all 

 Cephalopoda, or* veins. If on the otter hand a greater histological 



differentiation corresponding to that which exists in the Vertcbrata be 

 required to constitute vein, evidence of the existence of anything 

 of the kind in the greater proportion of the venous blood-channels of 

 these creatures is at present wanting. 



As regards the grosser structure of the circulatory apparatus in the 

 Molltuca proper, it may be observed that, in the Lamtllibranchiata 

 there is either single auricle and a single ventricle (Ottrtta), a single 

 ventricle and a double auricle (most Lamellibranchs), or two auricles 

 and two ventricles (Area). In all other Mollutca, except the Cephalo- 

 poda, there is single auricle and a single ventricle. In the 

 Cephalopoda the heart is essentially similar to that of the Lamelli- 

 branch", inasmuch as it consists (in the Dibranchiata) of a single 

 ventricle and of two contractile, so-called ' Bronchio-Cardiac Veins,' 

 which represent the two auricles of the Lamellibranchs. The circu- 

 lation in these creatures is assisted (at least in Loliyu media, in which 

 we lately had opportunities of convincing ourselves of the fact), not 

 only by the regular contraction of the so-colled ' branchial hearts,' 

 which are dilatations of the afferent branchial veins, but by that of 

 the gills themselves. 



The nature of the so-called Pericardium in the A/ullutca has been 

 much misunderstood. It is most important to recollect that in no 

 case is there evidence of its being a closed serous sac comparable to 

 the pericardium of the higher animals. On the contrary, wherever it 

 has been examined with sufficient care (Lamtllibranchiata, Pteropoda, 

 J/cteropoda, Nudibranehiata, and Cephalopoda), it has been found to 

 be a blood-sinus, which in some cases (Pteropoda, Cephalopoda ('.), 

 Lamcllibranchiata(i), and J/eteropoda) communicates with the ex- 

 terior by the mediation of the renal organ. 



3. The Respiratory Function is performed by modifications of several 

 distinct parts in the Molliuca. 1. By the general surface of the 

 pallial cavity, which may be more or less adaptively modified : this 

 kind of respiratory organ is to be found in the Brachiopoda, Ptero- 

 poda, and Pulmonata. 2. By specially modified parts of the walls of 

 the pallial cavity into true gills : the whole tendency of the modifica- 

 tion of form which these gills undergo ia to increase their surface, 

 and this end, generally speaking, is effected in one of three ways : 



a. By the development of simple processes, as in Patella or Atlanta. 



b. The simple processes become ramified, so that the gill eventually 

 consists of a stem with lateral branches, and these again may be sub- 

 divided into smaller and smaller branchleta Pectinibranchiata and 

 Cephalopoda, c. In the Lamcllibranchiata each gill essentially consists 

 of a stem with lateral undivided branches, and in such forms aa 

 Trigonia and ffucnla (Jig. 4, No. 3, t.) ; the branchiae have precisely this 

 structure. In Nucula the lateral branches are comparatively short, 

 but in Trigonia they are much longer. In Pectea they turn up at 

 their free ends upon themselves and form a close loop, so that the 

 Tree end takes a position near the fixed extremity ; at the same time 

 lateral processes are given off from the branches which unite and 

 connect them together by a very loose and open vascular network. 

 Each gill has thus become a flattened jR>uch, completely open, 

 both laterally and superiorly ; the sides of the pouches are very 

 open, and are constituted superficially by the parallel produoed 

 and reflected portions of the gill -branches, and more deeply by the 

 very loose network formed by the anastomosing lateral processes. 

 Now, if we suppose that the reflected portion of the outer gill-pouch 

 adheres to the mantle, while the reflected portion of the inner gill- 

 pouch remains free on each side of the foot, but adheres to its fellow 

 behind the foot, thus forming a complete partition across the pallial 

 cavity, the deep vascular network becoming very close, and giving 

 off vertical septa, by which tho pouch becomes divided into successive 

 antero-posterior chambers ; then the result will be such a gill as we 

 meet with in the Oyster, the Unto, and the great majority of IMUI.UI- 

 branchiata. The minute structure of these bronchia! strikingly 

 resembles that of the branchial sac of the Ascidians, as has been long 

 since pointed out by Siebold and others, and has given rise to the 

 prevalent idea that the two organs are homologous. Structural 

 resemblance, however, is in itself no true basis for the establishment 

 of homologies, and here there are abundant means of demonstrating 

 the resemblance to be simply analogical. 3. The ' branchial' of the 

 Xudiliranchiala again doubtless subserve respiration, but they are 

 developed from the mesosoma, and contain the gastro-hepatic processes 

 of the alimentary canal features by which they are essentially 

 distinguished from true gills. 4. The branchial sac of the Ascidiaus 

 is, as we have shown, a modification of their pharyngeal sac, resembling 

 the gills of fishes (especially Amiihionu) more than any structure to 

 be found in other Jnrcrtebrala (the nearest approximation perhaps is 

 in the cloacal bronchia: of Neuropterous Larvtc and of some Annelida). 

 Like the wall of the gill-pouch of Lamcllibranchiata, that of tho 

 branchial sac of the Ascidians is fundamentally composed of two ele- 

 ments a superficial strong framework of branchial bars corresponding 

 with the 'gill-branches,' and a deeper vascular network connecting these. 

 The more obvious peculiarities in the structure of the branchial sac 

 of Ascidiaus are produced by the plaiting of its wall into tho so-called 

 branchial folds, which may vary in number from four (Cynthia) to a 

 number so great that tho wall of the sac appears crimped (Phattutla). 



4. The Renal Organs. The existence of a special organ fur the urinary 

 secretion has now been demonstrated in all the great divisions of the 

 iloUtuca except the Polytoa and Vrachiopoda. The essential feature 



