186 PHYLUM ECHINODEBMATA. 



are of the nature of lymphatic glands, and that their lining 

 membrane which has the form of a connective tissue network 

 containing dividing cells in its meshes buds off amoeboid 

 cells which enter the fluid of the system. 



The axial organ (ovoid gland, heart) is a fold of the wall of 

 the axial sinus, into which it projects. Its walls are secondarily 

 folded so that it appears in section to contain prolongations of 

 the axial sinus. It consists mainly of connective tissue and of 

 cells derived from a prolongation of the generative rudiment 

 (pp. 131, 146). 



The so-called vascular system (lacunar system) of Asteroids 

 consists of tracts of connective tissue in which the fibres are 

 sparser, the ground substance stains more deeply and the 

 leucocytes are more numerous than in the ordinary connective 

 tissue. There is a cord of it in the vertical septum dividing the 

 radial perihaemal canal (Fig. 129, 3) : this is the so-called radial 

 blood-vessel. This is continuous with a circumoral cord of 

 the same tissue (Fig. 135, B). There is a certain amount of it 

 in the wall of the stomach, which presumably is in connexion 

 with the circumoral tract, and it is possible that the latter may 

 send prolongations on to the axial organ. There is also a tract 

 of it in close association with the generative rachis in the wall 

 of the aboral sinus. 



The vascular tissue is sometimes described as consisting of bundles of 

 anastomosing canals without any epithelial lining and containing a coagul- 

 able fluid. A definite circulation of this fluid has never been observed. 



The sexes are separate. Asterina gibbosa however has been 

 said * to be a protandrous hermaphrodite, the same gland pro- 

 ducing in young specimens spermatazoa and in old ones ova. 



The generative organs consist of bunches of tubes which are 

 attached on each side of the interbrachial septa to the abactinal 

 body -wall (Fig. 127). There are therefore twice as many gonads 

 as there are arms. Each gonad consists of one or of several 

 tufts of tubes. In the latter case, the gonads extend into the 

 arms, to the dorso-lateral wall of which they are attached. 

 Each gonad opens to the exterior by one simple opening (rarely 

 subdivided into several) <in the abactinal wall of the disc, or, 

 when there are several gonadial tufts, by as many pores as 



* Cuenot, L. Arch. Zool. Exp. et gen. 5 bis, 1888. 



