80 



INTRODUCTION TO CLASSIFICATION. 



of which either a separate offspring arises, or an adult is 

 developed within an embryonic form ; but in these cases the 



Fig. 39. 





Fig. 39. Pilidium gyrans (after Leuckart and Pagens lecher). 



A. Young Pilidium; a, alimentary canal ; b, rudiment of the Nemertid. 

 li. Pilidium with a more advanced Nemertid. 

 C. Newly-freed Nemertid. 



appropriation of the intestine of the primary by that of the 

 secondary form, which renders the ordinary development of the 

 Echinoderrn so striking, does not occur. 



In discussing the characters of the Echinodermata, I have 

 described at length the ambulacral system ; and, in speaking 

 of the Scolecida, I have no less insisted upon the peculiarities of 

 the " water- vascular system." But it is impossible to compare 

 these two systems of vessels without being struck by their 



