THE ANNULOIDA. 77 



vessels which attains so large a development as the " water- 

 vascular " apparatus in many Scolecids. 



Whatever value may be attached to these resemblances, it 

 must, I think, be admitted that, in the present state of our 

 knowledge, it is impossible to affirm anything absolutely common 

 to, and yet diagnostic of, all Annidosa and all Echinoderms and 

 Scolecids. On the other hand, there can be no doubt as to the 

 many and singular resemblances which unite the Scolecids and 

 the Echinoderms together. The nervous system of the Echi- 

 noderm may present considerable differences from that of a 

 Trematode or Rotifer, but it must be recollected that the com- 

 parison is not a fair one, seeing that the mouth and gullet of an 

 Echinoderm, round which its nervous ganglia are arranged, are 

 not, strictly speaking, the same as the parts so named in a 

 Rotifer, but are new developments. 



And it is exactly in that anomalous method of development 

 of the Echinoderm within its larva, which is so characteristic 

 of the whole group of Echinodermata, that this class exhibits its 

 strong alliance with the Scolecida ; the Turbellaria and Tseniada 

 exhibiting the only approach to the method of Echinoderm 

 development known in the xlnimal Kingdom. 



A singular larva studied by Johannes Miiller, in one of his 

 many fruitful visits to the seashore, and termed by him Pilidium, 

 has furnished, in the hands of subsequent observers (more espe- 

 cially Krohn, Leuckart, and Pagenstecher), ample proof that 

 a Nemertes (a genus of Turbellaria) may be developed in a 

 manner altogether similar to that in which an Echinoderm 

 takes its origin. 



The Pilidium (Fig. 37) is a small, helmet-shaped larva, 

 with a long flagellum attached like a plume to the summit of 

 the helmet, the edges and side lobes of which are richly ciliated. 

 A simple alimentary sac opens upon the under surface of the 

 body between the lobes (Fig. 37, a). 



In this condition, the larva swinis about freely ; but, after a 

 while, a mass of formative matter appears upon one side of 

 the alimentary canal, and, elongating gradually, takes on a 

 worm-like figure. Eventually it grows round the alimentary 

 canal, and, appropriating it, detaches itself from the Pilidium 



