293 



PERIHiEMAL AND PSEUDOHiEMAL SYSTEMS 



The radial water vessel in each arm lies close under the 

 ambulacral ossicles ; below it there is a coelomic space, roughly 

 diamond-shaped in transverse section, which is known as the 

 radial perihaemal cavity. Below the perihaemal cavity the epidermis 

 is thickened by an increase in the nerve plexus, and folded so as 

 to project into the ambulacral groove as the nerve ridge. Round 

 the mouth, the radial perihaemal vessels are joined by an oral 

 perihaemal ring.^ Each radial vessel is divided longitudinally by 

 a vertical septum, and in this septum lies a strand of a peculiar 

 tissue which in the starfish takes the place of the blood vessels. 

 This is a part of the connective tissue in which the fibres are more 

 sparse and the ground substance more fluid than elsewhere, and 

 it is believed that along the strands which are formed of it sub- 

 stances diffuse, and amoeboid cells wander, more readily than 

 elsewhere. Around the mouth a ring strand joins the radial 

 strands, with this is connected the tissue of the axial organ, and 

 with the aboral end of the axial organ is again connected an aboral 

 ring, from which strands extend to the generative organs. 



REPRODUCTION 



The axial organ, however, is primarily of importance, not as 

 a part of this ' pseudohaemal system ', but as the original seat 

 of the genital cells, for which reason it is often known as the 

 * genital stolon ', while the aboral ring is the ' genital rachis '. 

 Along the latter the genital cells wander from the stolon to the 

 actual gonads. These are ten in number, shaped like bunches 

 of grapes, and varying in size with the season of the year. They 

 are attached to the body-wall by their ducts, which open one 

 on each side of the base of each arm, towards the oral aspect. 

 The sexes are separate, but do not differ externally. Eggs and 

 sperms are shed into the water, where fertilisation takes place. 

 The cleavage of the ovum is complete (holoblastic, p. 644) and 

 practically equal. It leads to the formation of a remarkable, 

 bilaterally symmetrical larva (the bipinnaria. Fig. 220), which 

 swims by two bands of ciha. This, after passing through a fixed 

 stage (Fig. 221), gives rise to the radially symmetrical adult, 



1 Adjoining this is another coelomic tube, the so-called ' inner perihaemal ring '. 

 which is connected not with the perihaemal vessels but with the axial smus. 



