3. They receive water through their mouth, opening the opercula, and emit it again by the upper passage of the mouth, closing the opercula by which it entered; these are immediately beneath the eyes, and when they take in water they close the mouth with both opercula, and thus eject again the sea-water. Next to the teeth is a very short œsophagus, so that the mouth appears joined to the stomach, and from this proceeds a divided stomach, from the middle of which is a single thin intestine; this intestine ends externally beneath the folding of the extremity, as I said before. Between the opercula there is something resembling the appendages to the teeth of the carabi; within the abdomen is an ochreous chyme, and some small elongated white bodies, and other red ones scattered through it. The male differs from the female in length and width, and in the abdominal covering, for this is longer in the female, farther from the body, and more thick-set with appendages, as in the female carabi. The parts of the malacostraca are of this nature. CHAPTER IV. 1. The testacea, as cochleæ,[120] and cochli,[121] and all that are called ostrea,[122] and the family of echini, are composed of flesh, and this flesh is like that of the malacostraci, for it is internal; but the shell is external, and they have no hard internal part. But they have many differences amongst themselves, both in regard to their external shells and their internal flesh, for some of them have no flesh at all, as the echinus; in others it is entirely internal and out of sight, except the head, as the land snails and those called coccalia,[123] and in the sea the purpura[124] and the ceryx,[125] the cochlus, and all the turbinated shells. [120] Land snails. [121] Marine. [122] Bivalves. [123] Some small land snail with a conical shell. [124] Purpura. [125] Whelk. 2. Of the rest some are bivalves, others univalves. I call those bivalves which are enclosed in two shells; the univalves are enclosed in one shell, and the fleshy part is uncovered, as the lepas.[126] Some of the bivalves can open, as the pectens and mya, for all these are joined on one side, and separated on the other, so as to shut and open. There are other bivalves which are joined on both sides, as the solen; others which are entirely enclosed in their shells, and have no external naked flesh, as those which are called tethya.[127] [126] Patella, limpet. [127] Ascidians.