48 



INTRODUCTION TO CLASSIFICATION. 



cilia. The Trematode and Tsenioid worms have a similar, but 

 usually much more ramified apparatus ; and it is interesting to 

 observe that, in these animals, as in the Aspidogaster conchicola 

 (Fig. 22), the water-vascular system becomes divided into two 



Fig. 22. 



Fig. 22. A. Water-vascular system of Aspidogaster conchicola ; a, terminal pore ; 

 b, lateral contractile vessels ; c, lateral ciliated trunks, that of the left side shaded ; 

 d, dilatation of this trunk ; B, one of the larger, and C, one of the smaller, ciliated 

 vessels. 



distinct portions, one with contractile and non-ciliated walls, 

 the other with non-contractile and ciliated walls. In some 

 Turbellaria the apertures of the water-vascular apparatus are 

 multiple ; while it would seem that in others, as the Nemertidse, 

 the apparatus becomes shut externally in the adult state, and 

 consists mainly, if not exclusively, of contractile vessels. The 

 difficulties of observation are here, however, very great, and 

 I would be understood to express this opinion with all due 

 caution. 



In none of these animals has any other set of vessels than 

 those which appertain to the water- vascular system (if I am 

 right in my view of the vessels of the Nemertidse) been ob- 

 served, nor has any trace of a true heart been noticed. The 

 nervous system consists of one, or two, closely approximated 

 ganglia. 



This sum of common characters appears to me to demand 

 the union of the Rotifera, Turbellaria, Trematoda, and Tseniada 

 into one great assemblage. Ought the Nematoid worms to be 

 grouped with them ? If the system of canals, in some cases 

 contractile, which open externally near the anterior part of 



