tiated, the cells of the latter give rise by division to two bands of cells, which lie, one on each side of the long axis of the ventral face of the worm, and constitute the meso- blast. This becomes marked out by transverse constric tion into segments, and, in each segment, gives rise to all the tissues which lie between the epiblast (epidermis) and lypoblast (epithelium of the alimentary canal). The mouth corresponds with the primitive involution of the Morula ; the anal aperture is a new formation. In the Nematoidea and in the lower Rhabdoccele Turbellaria, the intestinal canal is a simple tube or sac. But in some Turbellaria and Trematoda, the alimentary canal gives off diverticula, which ramify through the mesoblast and even unite to gether. The like takes place in a great many Ccelenterata, and the " gastrovascular apparatus," as it has been well termed, which is thus formed, is highly characteristic of them. The animals just referred to, therefore, have an " entero- ccele" more or less distinct from the proper digestive cavity,
but connected with it, and ramifying through the mesoblast.2. In the remaining members of the animal kingdom, the -embryo develops a secondary mouth as a perforation of the body wall, the primary aperture sometimes becoming the anus, and sometimes disappearing. Of these Metazoa deuterostomata, there are some which follow the mode of development of the Oli jochceta and Hirudinea very closely, so far as the formation and segmentation of the mesoblast is concerned, though the question, whether this segmented mesoblast arises from the epiblast or the hypoblast, has not been exhaustively worked out. These are the Anne lida polychceta ; and there is the closest resemblance in development between them and the lower Arthropoda (Crustacea, Arachnida, lower Insecta), while, in the higher Arthropods, the process is complicated by the development 3f an amnion, and by some other special peculiarities which need not be considered in detail. In all these Metazoa, whatever cavities are developed in the mesoblast, whether a wide peri visceral cavity, or vascular canals, or both combined, they arise from the splitting or excavation of tho mesoblast itself, and are not prolongations of the alimentary cavity. Hence they may be termed Schizoccela.
But, in certain other deuterostomatous Metazoa, the meso blast becomes excavated, and a " perivisceral cavity " and vessels are formed in quite another fashion.
Thus in the Chcetognatha, represented by the strange and apparently anomalous Sagitta, Kowalewsky s researches show, that the vitellus undergoes complete segmentation, and is converted into a vesicular Morida, on one side of which involution takes place, and gives rise to the primi tive alimentary canal, of which the opening of involution becomes the permanent anus, the mouth being formed by perforation at the opposite end of the body. Before the mouth is formed, however, the primitive alimentary cavity throws out, on each side, a csecal pouch, which extends as far forwards as its central continuation does, and grows backwards behind the anus. The two sacs, thus prolonged posteriorly, meet, but remain divided from one another by their applied walls in the median line. These lateral sacs now become shut off from the median portion of the primi tive alimentary cavity (which opens at its anterior end, and becomes the permanent alimentary canal), and are con verted into shut sacs, the cavity of each of which forms one-half of the perivisceral cavity. The inner wall of each sac, applied to the hypoblast, gives rise to the muscular wall of the intestine ; and the outer wall, applied to the epiblast, becomes the muscular wall of the body, and from it the generative organs are evolved. The great ganglia and nerves are developed from the cells of the epiblast. Thus tSagitta is temporarily ccelenterate, but the two gastrovas cular sacs, each enclosing an enteroccele, become shut off from the alimentary canal and metamorphosed into the walls of the perivisceral cavity. But it is not altogether clear whether the cells of the enterocoele give rise only to the lining of the perivisceral cavity, and whether the muscles and connective tissue are otherwise derived or not. Kowalewsky s evidence, however, is in favour of the origin of the muscles directly from the cells of the mesoblastic diverticula.
In the Echinodermata, the brilliant investigations of Johannes Miiller, confirmed in their general features by all subsequent observers, proved, firstly, that the ciliated embryonic Gastnda (the primitive alimentary canal of which is formed by involution of a vesicular blastoderm), to> which the egg of all ordinary Echinoderms gives rise, acquires a mouth, by the formation of an aperture in the body wall, distinct from the primitive aperture of the Gastrula, so that, in this respect, it differs from the embryo of all Ccelenterata; secondly, that the embryo thus pro vided with mouth, stomach, intestine, and anus acquires a complete bilateral symmetry ; thirdly, that the cilia, with which it is primitively covered, are ultimately restricted to one or more series, some of which encircle the axis of the body, or a line drawn from the oral to the anal aper tures ; and fourthly, that, within this bilaterally sym metrical larva or Echinopaidiiim, as it may be called, the more or less completely radiate Echinoderm is developed by a process of internal modification.
Miiller believed that the first step in this process was the ingrowth of a diverticulum of the integument, as a hollow process, which became converted into the ambulacral vascular system of the Echinoderm. He did not attempt to explain the origin of the so-called blood-vascular system, or pseudhoemal vessels, nor of the perivisceral cavity. Miiller s conclusions remained unchallenged until 1SG4, when Prof. Alexander Agassiz took up the question afresh, and, in a remarkable paper on the development of the genus Asteracanthion, detailed the observations which led him to believe that the ambulacral vessels do not arise by involution of the external integument, but that they com mence as two primitively symmetrical diverticula of the stomach (the " wiirstf ormige Korper" of Miiller), one of which becomes connected with the exterior by an opening (the dorsal pore observed by Miiller, and considered by him to be the origin of the ambulacral vessels), and gives- rise to the ambulacral vessels, the ambulacral region of the body of the Echinoderm being modelled upon it; while upon the other gastric sac, the antambulacral wall of the starfish body is similarly modelled. Both gastric sacs early become completely separated from the stomach of the Echinopocdium, and open into one another, so as to form a single horseshoe-shaped sac, connected with the exterior by a tube which is converted into the madreporic canal. Agassiz does not explain the mode of formation of the perivisceral cavity of the starfish, and has nothing to say respecting the origin of the pscudhsenial vessels.
Recently, Metschnikoff has confirmed the observations of Agassiz, so far as the development of the ambulacral system from one of the diverticula of the alimentary canal of the starfish larva is concerned, and he has added the important discovery that the perivisceral cavity of the Echinoderm is the product of what remains of these diver ticula. Moreover, his observations on other Echinodermata. show that essentially the same process of development of the peritoneal cavity occurs in Ophiuridce, Echinidce, and Holotkuridce.
The precise mode of origin of the pseudhsemal system, or so-called blood-vessels, of the Echinoderms is not yet made out. But it is known that the cavity of these vessel* contains corpuscles similar to those which are found in. the perivisceral cavity and in the ambulacral vessels, and that all three communicate.
