554 



INVERTEBRATE MORPHOLOGY. 



lime imbedded in the connective tissue, and bear upon their 

 free extremity a number of radiating spines, which vary in 

 the amount of movement of which they are capable in differ- 

 ent species. The paxillae are frequently found in groups 

 around the dermal branchiae, over which the spines may be 

 bent so as to serve for protection. 



These dermal branchiae (Fig. 254, h) are pouchlike evagi- 

 nations of the coelomic cavity with thin walls composed of 

 ectoderm and a layer of ciliated cells continuous with the 

 peritoneal lining of the coelom, between these two layers 

 there being but a slight development of connectivetissue and 



Fig. 254.— Transverse Section of Arm of a Starfish (modified from 



LUDWIG). 



A = ambulacral plate. I = schizocoelic sinus. 



am = ampulla. mn — muscular nervous system. 



an = aboral nerve. N = epithelial nervous system. 



= ovum. 



p = peritoneal epithelium. 

 pi = calcareous plate. 

 rh = radial hydroooel-vessel. 



B = adambulacral plate. 



b = branchia. 



c = digestive caecum. 

 ec = ectoderm. 



if = tube-foot. 



circular and longitudinal muscle-fibres. These pouches are 

 scattered plentifully over the aboral surface, and in some 

 forms occur upon the oral surface also. Their thin walls and 

 the extent of surface they collectively represent leave little 

 room for doubt but that they possess respiratory functions 

 though they may also serve indirectly in excretion, since it has 

 been asserted that the amoeboid cells of the coelomic hsemo 



