554 



INVERTEBRATE MORPHOLOGY. 



lime imbedded iu 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 paxillse 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, b) are pouchlike evagi- 

 natious of the coalomic cavity with thin walls composed of 

 ectoderm and a layer of ciliated cells continuous with the 

 peritoneal lining of the coeloni. between these two layers 

 there being but a slight development of connective tissue and 



an 



ec 



am 



w^,^ { "\y^^|piMiilP^ 



B A N tr 



FIG. 254. TRANSVERSE SECTION OP AUM OP A STARFISH (modified from 



LUDWIG). 



A -- ambulacra! plate. 

 am = ampulla. 

 an = aboral nerve. 

 B = adambulacral plate. 

 b = branchia. 

 c = digestive caecum. 

 ec = ectoderm. 



I = schizoccelic sinus. 

 mn = muscular nervous system. 

 2? = epithelial nervous system. 

 o = ovum. 



p = peritoneal epithelium. 

 pi = calcareous plate. 

 rh = radial hydrocoel-vessel. 



tf 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 coslomic haemo- 



