MOLLUSCA 



97 



very generally an outer circular layer (i.e., a layer in which, 

 the muscular fibres run in a direction transverse to the 

 long axis of the body) and a deeper longitudinal layer ; 

 to these oblique and radiating fibres may be added. The 

 splanchnic musculature, though more delicate, exhibits a 

 circular layer nearer the enteric epithelium, and a longi- 

 tudinal layer nearer the coelomic surface. 



In Ccelomata and in many Coelentera there are found 

 distributed between the tracts of muscular tissue, bounding 

 them and giving strength and consistency also to the walls 

 of the body, of the alimentary canal, of the coelom, and of 

 the various organs and tissue-masses (such as nerve-centres, 

 gonads, &c.) connected with these, tracts of tissue the 

 function of which is skeletal. The skeletal tissue of 

 MoUusca, in common with that of other Coelomata, exhibits 

 a wide range of minute structure, and is of differing density 

 in various parts ; it may be fibrous, membranous, or carti- 

 laginous. The Mollusca, in common with the other Coelo- 

 mata, exhibit a remarkable kind of association between the 

 various forms of skeletal tissue and the epithelium which 

 lines the coelomic cavity. The coelomic cavity contains a 

 liquid which is albuminous in chemical composition (blood- 

 lymph or HiEMOLYMPH), and into this liquid cells are shed 

 from the coelomic epithelium. They float therein and are 

 known as blood corpuscles or lymph coepuscles. The 

 coelomic space with its contained haemolymph is not usually 

 in Coelomata, and is not in Mollusca, a simple even-walled 

 cavity, but is broken up into numerous passages and re- 

 cesses by the outgrowths, both of the alimentary canal and 

 of its own walls. By the adhesion of its opposite walls, 

 and by an irregularity in the process of increase of its area 

 during growth, the coelom becomes to a very large extent a 

 spongy system of intercommunicating LACUNiB or irregular 

 spaces, filled with the coelomic fluid. At the same time, the 

 coelomic space has a tendency to push its way in the form of 

 narrow canals and sinuses between the layers of skeletal tissue, 

 and thus to permeate together with the skeletal tissue in 

 the form of a spongy, or it may be a tubular, network all 

 the apparently solid portions of the animal body. This 

 association of the nutritive and skeletal functions is accom- 

 panied by a complete identity of the tissues concerned in 

 these functions. Not only is there complete gradation 

 from one variety of skeletal tissue to another (e.ff., from 

 membranous to fibrous, and from fibrous to cartilaginous) 

 even in respect of the form of the cells and their intercellular 

 substance, but the coelomic epithelium, and consequently 

 the haemolymph with its floating corpuscles derived from 

 that epithelium, is brought into the same continuity. The 

 skeletal and blood-containing and -producing tissues in fact 

 form one widely- varying but continuous whole, which may 

 be called the skbleto-teophic system op tissues. 



In many Coelomata. not only do the skeletal tissues 

 allow the coelomic space with its fluid and corpuscles to 

 penetrate between their layers, but a special mode of 

 extension of that space is found, which consists in the 

 hollowing out of the soUd substance of elongated cells 

 having the form of flbres, which thus become tubular, 

 and, admitting the nutritive fluid, serve as channels for 

 its distribution. These are " capillary vessels," and it has 

 yet to be shown that such are formed in the Mollusca. 

 Larger vessels, however, concerned in guiding the move- 

 ment of the coelomic fluid in special directions are very 

 usually developed in the Mollusca, as in other Coelomata, 

 by the growth of skeletal tissue around what are at first 

 ill-defined extensions of the coelomic space. In this way 

 a portion of the coelomic space becomes converted into 

 vessels, whilst a large part remains with irregular walls 

 extending in every direction between the skeletal tissues 

 and freely communicating with the system of vessels. As 

 in many other Coelomata, muscular tissue grows around 



the largest vessel formed from the primitive coelom, which 

 thus becomes a contractile organ for propelling the blood- 

 lymph fluid. This " heart " has in Mollusca, as in most 

 other Coelomata in which it is developed, a dorsal position. 

 A communication of the blood-lymph space with the 

 exterior by means of a pore situated in the foot or else- 

 where has been very generally asserted to be chajjacteristic 

 of Mollusca. It has been maintained that water is intro- 

 duced by such a pore into the blood, or admitted into a 

 special series of water-vessels. It has also been asserted 

 that the blood-fluid is expelled by the Mollusca from these 

 same pores. Recent investigation (14) has, however, made 

 it probable that the pores are the pores of secreting glands, 

 and do not lead into the vascular system. There is, it there- 

 fore appears, no admission or expulsion of water through 

 such pores in connexion with the blood, although in some 

 other Coelomata it is established that water is taken into 

 the ccelomic space through a pore (Echinoderms), whilst in 

 some others there is no doubt that the coelomic -haemolymph 

 is occasionally discharged in quantity through pores of defl- 

 nite size and character (Earthworm, &c.). 



We have thus seen that the Mollusca possess, in common 

 with the other Coelomata — 1, a body composed of a vast 

 number of cells or plastids, arranged so as to form a sac- 

 Uke body-wall, and within that a second sac, the met-enteron, 

 the wall of which is separated from the first by a coelom or 

 blood-lymph space ; 2, a stomodxum and a proctodeum ; 

 3, a prostomium, together with a differentiated dorsal and 

 ventral surface, and consequently right and left sides, i.e., 

 bilateral symmetry ; 4, a pair of nephridia ; 5, gonads 

 developed on the wall of the coelom ; 6, deric epithelium 

 (producing horny and calcareous deposits on its surface), 

 enteric epithelium, and coelomic epitheUmn; 7, laterally 

 paired masses of nerve-tissue, especially large in the pro- 

 stomial region (nerve-centres or- ganglia) ; 8, muscular 

 tissue, forming a somatic tunic and a splanchnic tunic ; 9, 

 skeleto-trophic tissues, consisting of membranous, fibrous, and 

 cartilaginous supporting tissues, and of blood-vessels and the 

 walls of blood-spaces, the coelomic epithelium, and the liquid 

 tissue known as hxmolymph (commonly blood). 



Schematic Mollusc. — Starting from this basis of structural 

 features common to them and the rest of the Coelomata, 

 we may now point out what are the peculiar developments 

 of structure which characterize the Mollusca and lead to 

 the inference that they are members of one peculiar branch 

 or phylum of the animal pedigree. In attempting thus to 

 set forth the dominating structural attributes of a great 

 group of organisms it is not possible to make use of arbi- 

 trary definitions. Of Mollusca, as of other great phyla, it 

 is not possible categorically to enunciate a series of struc- 

 tural peculiarities which will be found to be true in refer- 

 ence to every member of the group. We have to remember 

 that the process of adaptation in the course of long ages 

 of development has removed in some cases one, in other 

 cases another, of the original features characteristic of the 

 ancestors from which the whole group may be supposed to 

 have taken origin, and that it is possible (and actually is 

 realized in fact) that some organisms may have lost all the 

 primary characteristics of MoUuscan organization, and yet 

 be beyond all doubt definitely stamped as Mollusca by 

 the retention of some secondary characteristic which is so 

 peculiar as to prove their relationship with other Mollusca. 

 An example in point is found in the curious fish-Uke form 

 Phyllirhoe (fig. 58), which has none of the primary char- 

 acteristics of a Mollusc, and yet is indisputably proved to 

 belong to the MoUuscan phylum by possessing the peculiar 

 and elaborate lingual apparatus present in one branch of 

 the phylum, the Glossophora. 



In order to exhibit concisely the peculiarities of organi- 

 zation which characterize the Mollusca, we find it most 



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