. Text-book of embryology. Embryology. XTI ECHINODEEMATA 475 outgrowth from this coelom extends. It is noteworthy that, counting the arms from before backward, No. 1 arm is really situated over No. 2 lobe of the hydrocoele, and eventually fuses with it; and later in the metamorphosis, when the ring-shaped growth of the left hydrocoele and the left posterior coelom is complete. No. 1 hydro- coele lobe comes to lie under No. 5 arm. The neighbouring angles of adjacent perihaemal spaces grow into the arms beneath the hydrocoele lobe, and in this way the two radial perihaemal canals, which are foun
. Text-book of embryology. Embryology. XTI ECHINODEEMATA 475 outgrowth from this coelom extends. It is noteworthy that, counting the arms from before backward, No. 1 arm is really situated over No. 2 lobe of the hydrocoele, and eventually fuses with it; and later in the metamorphosis, when the ring-shaped growth of the left hydrocoele and the left posterior coelom is complete. No. 1 hydro- coele lobe comes to lie under No. 5 arm. The neighbouring angles of adjacent perihaemal spaces grow into the arms beneath the hydrocoele lobe, and in this way the two radial perihaemal canals, which are found in each adult arm, are formed. The external peri- haemal ring-canal is formed by the fusion of the main portions of these spaces. The internal perihaemal canal is formed by a circular extension of the hinder part of the anterior coelom which is included within the. Fig. 365.—Longitudinal frontal sections of larvae otAsierina gibbosa, to show the segmenta- tion of the coelom and the origin of the hydrocoele and madreporic vesicles. (Original.) A, section of larva about five days old. B, section of larva about six days old. C, section of larva about six and a half to seven days old. ae, anterior coelom; ^, rudiment of adult stomach; hy, rudi- ment of the hydrocoele ; 1, 2, etc., its lobes; , left po-sterior coelom; , madreporic vesicle; , rudiment of peri-oral coelom. , right posterior coelom ; st, larval stomach. body of the star-fish when the stalk finally disappears. This portion of the anterior coelom, into which pore-canal and stone-canal open, is known as the axial sinus (Fig. 366, ah^). The septum, which divides it from the general body-cavity surrounding the stomach in the adult star-fish, is nothing but the old transverse septum which separated the anterior coelom from the left posterior coelom in the larva; audit follows, therefore, that the general body-cavity of the adult is only the ring-shaped left posterior coelom, which, with Goto, we may
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