XIV] ASTEKIAS 337 



germs of sessile animals and plants. They cover the thickened 

 bases of the blunt spines with which the back is beset. Larger 

 pedicellariae are scattered in the interspaces between the spines 

 and are distinguished from the smaller by the fact that the blades 

 do not cross as is the case with these. The larger kind are also 

 found on the adambulacral spines. 



The pedicellariae are probably little spines of the second order. 

 In the small blunt-armed star-fish, Asterina gibbosa, there are no 

 true pedicellariae, but the plates on the back bear small spines 

 arranged in twos or threes, which act somewhat like pedicellariae 

 when the skin is irritated. 



The dermal branchiae (5, Fig. 153) are conspicuous in a 

 star-fish when alive; they are very difficult, on the 



Branchiae. * . * 



other hand, to detect in preserved specimens. They 

 are in fact thin spots on the body-wall, where it consists only of 

 the ectoderm and the wall of the coelom closely apposed, the 

 jelly, fibres and skeletal rods being absent. These spots project 

 like little finger-shaped processes and their purpose is to facilitate 

 respiration. The fluid in the coelom or body-cavity being here 

 separated from the external water by a very thin membrane, the 

 dissolved oxygen is able to pass from the one fluid to the other 

 with great ease. 



There is no localised excretory organ in the star-fish or indeed 

 in any Echinoderm. Throughout the phylum so far as is known 

 this function is performed by the amoebocytes which float in the 

 coelomic fluid and have been produced by the budding of the cells 

 forming the wall of the coelom. When charged with excreta the 

 amoebocytes endeavour to make their way out. This in the star-fish 

 they effect by accumulating at the base of the dermal branchiae 

 and working their way through the thin body-wall and so escaping 

 into the ocean. 



The organs of sex in the star-fish are very simple. Both kinds 

 Reproductive of germ cell are aggregated in great feather-shaped 

 organs. glands situated in pairs in the bases of the arms and 



opening in the angles between the arms or in the interradii. The 

 ten ovaries in the female and ten testes in the male are connected 

 by a circular cord of immature germ cells called the genital 

 rachis running round the disc just dorsal to the coelom. This is 

 embedded in the wall of a tube called the aboral sinus which like 

 the other spaces in a star-fish, apart from those of the digestive 

 canal, is an outgrowth of the coelom. The rachis is in turn 

 8. & M. 22 



