XVI 



ECHINODERMATA 



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 



Ipc 



FIG. 365. Longitudinal frontal sections of larvae of Aster ina gibbosa, to show the segmenta- 

 tion of the coelom and the origin of the hydrocoele and madreporic vesicles. (Original.) 

 A, section of larva about live days old. B, section of larva about six days old. C, section of larva 

 about six and a half to seven days old. oc, anterior coelom ; a.st, rudiment of adult stomach ; hy, rudi- 

 ment of the hydrocoele ; 1, 2, etc., its lobes; Lp.c, left posterior coelom; m.v, madreporic vesicle; p.o.c, 

 rudiment of peri-oral coelom. r.p.c, 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, aV). 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 ; and it 

 follows, therefore, that the general body-cavity of the adult is only 

 the ring-shaped left posterior coelom, which, with Goto, we may term 

 the hypogastric coelom. The right posterior coelom of the larva 

 becomes the epigastric coelom of the adult. 



In Asterias, Goto maintains that the perihaemal spaces appear as 

 solid masses of mesenchyme, lying on the ventral surfaces of the arms 

 when metamorphosis is complete, and that these spaces subsequently 



