266 



THE STELLEROIDEA 



o.b. 



FIG. XVIII. 



valves. There are no suckers, so the podia are useless in loco- 

 motion. 



The Nervous System of Ophiura dliaris consists of a circum- 

 oesophageal ring, from which radiate five radial nerve trunks (Fig. 



XVII. c.ce.n and r.n) ; it also gives 

 oft' small branches to the oral ten- 

 tacles, teeth, and oral muscles. The 

 radial nerve trunks consist of two 

 separate nerve bands, one above the 

 other, both lying in the " epineural 

 canal " (Fig. XXI. e.r.n and i.r.n). 

 The two nerve bands are not con- 

 nected, though situated very close 

 to one another. Both radial nerve 

 trunks thicken into ganglion -like 

 swellings, of which there is one in 

 each arm segment. From the ganglia 

 of the upper or internal nerve band, 

 branches are given off to the muscles 

 between the vertebral ossicles. 

 From each ganglion of the lower 

 water-vascular ring of Ophiura dliaris. or external nerve trunk, two pairs 



<>.b, branch to oral tentacles ; c.o.v, circuni- f 



oesophagcal vessel; m, inadreporite ; p. r, Of IiervCS are given OH ; One pair 



JlfSone'cSf. r '*' "* M "**" v ** 1 ' supplies the tentacles, and the other 



the integumont and spines. 



The last element in the Ophiuroid nerve system consists of a 

 "genital nerve ring," which lies along the "aboral circular sinus." 



Respiration in the Ophiuroids is effected only by the walls of 

 the bursae, and by the podia. 



The Reproductive Organs consist of a series of small pear- 

 shaped gonads (Fig. XIX. y) which do not open directly to the 

 exterior, but into the bursae, where, in some cases, the development 

 takes place. In Ophiura there are more than forty gonads to each 

 bursa. 



The Coelomic Sinus. There appears to be no true blood- 

 vascular system in the Ophiuroids ; the vessels which have been 

 described as such are connected with the axial sinus, and are there- 

 fore coelomic spaces and not vessels (MacBride, 32, and Russo). 

 The axial sinus (Fig. XX. x.s) is the most important ; it is the space 

 through which runs the stone-canal, and it contains a gland in con- 

 tact with the stone-canal, which is known as the " ovoid gland " or 

 "axial organ " (z.o), (see p. 23). From the upper end of the axial 

 canal runs the "aboral circular sinus." The course of this sinus 

 is very sinuous : above the arms it is aboral in position, but 

 beside the arms it bends downwards until, in the interradii, it 

 is quite ventral in position. The main sinus passes on the inter- 



