Cliaracteristics of the Echinodermata. cxliii 



in the Trematodes more than one asexual stage may interpose 

 itself between the stage of embryo and that of the perfect sexual 

 form. In the Cesfodes, the sexual zooids are retained for con- 

 siderable lengths of time attached to the asexual zooid, in the suc- 

 cessive aatero-posterior series in which they have been budded off 

 from its posterior extremity. Compound colonies are thus formed, 

 in which the setting free of the sexually perfect deuterozooid does 

 not entail the death of the ' nurse.' In the Trematodes the deutero- 

 zooids are contained within the body of the ' nurse/ and are only 

 set free by its disruption and death. Several of the forms of 

 reproduction observable in the Turbellaria, have been mentioned 

 above in the history of the Sub-kingdom Vermes. 



The Turbellaria possess a great power of repairing injuries and 

 mutilations^ and differ herein very markedly from the Discophorous 

 Annulata^ which have sometimes been classed with them. 



Sub-kingdom, Echinodermata. 



Animals which may be spheroidal or vermiform, star- or disc- 

 shaped, but which, whatever their external form, combine with a 

 radial and, ordinarily, pentamerous arrangement, traces of a bilateral 

 symmetry always detectible in developmental, and usually also in 

 adult life. The Sub-kingdom is divided into four Classes, the 

 Holothurioidea, the Echinoidea, the Asteroidea, and the Crinoidea. 

 The Class Asteroidea is divisible into two Sub-classes, the Asteriae 

 and the Ophiuridae. 



They possess a water-vascular system which surrounds the com- 

 mencement of their digestive tract, and, from the central ring 

 thus formed, sends out prolongations into the radii. Tubular pro- 

 cesses are developed upon these radial water-vascular stems in all 

 Echinodermata except the apodal Holothurioidea, and, except in 

 the Crinoidea, are used for the purpose of locomotion. The 

 water with which this system is distended finds its way into it 

 through the usually calcareous ' madreiwric" canal and ' tubercle,"' 

 which are in all Echinodermata, except the Crinoids, appended to 



