634 MOLLUSCA [SCHEMATIC MOLLUSC. 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 Mollusca, in common with that of other Ccelomata, 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 Ccelo- mata, exhibit a remarkable kind of association between the various forms of skeletal tissue and the epithelium which lines the ccelomic cavity. The ccelomic cavity contains a liquid which is albuminous in chemical composition (BLOOD- LYMPH or H^MOLYMPH), and into this liquid cells are shed from the coelomic epithelium. They float therein and are known as BLOOD CORPUSCLES or LYMPH CORPUSCLES. The coelomic space with its contained hsemolymph 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 LACUNAE or irregular spaces, filled with the coelomic fluid. At the same time, the ccelomic 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.g., 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 SKELETO-TKOPHIC SYSTEM OF TISSUES. In many Coelomata not only do the skeletal tissues allow the ccelomic 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 solid substance of elongated cells having the form of fibres, 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 ccelomic 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 Ccelomata, 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 characteristic 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. Kecent 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 Ccelomata 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 defi 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- like 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 proctodseum ; 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 epithelium ; 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 Ccelomata, 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 Molluscan 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-like form Phyllirhoe (fig. 58), which has none of the primary char acteristics of a Mollusc, and yet is indisputably proved to belong to the Molluscan 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