THE ANNULOSA AND ANNULOIDA. 79 



knowledge, it is impossible to affirm anything absolutely 

 common to, and yet diagnostic of, all Annulosa and all Echino- 

 derms and Scolecids. On the other hand, there can be no 

 doubt as to the many and singular resemblances which unite 

 the Scolecids and the Echinoderrns together. The nervous 

 system of the Echino'lerm may present considerable differences 

 from that of a Trematode or Eotifer, but it must be recollected 

 that the comparison 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 Eotifer, but are new developments. 



And it is exactly in that anomalous method of development 

 of the Ediinoclerin 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 Animal 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 especially Krohn, Leuckart, and Pagenstecher), ample 

 proof that a Nemertes (a genus of Tui-bellarici) may be developed 

 in a manner altogether similar to that in which an Echinoderm 

 takes its origin. 



The Pilidium (Fig. 39) 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. 39, A). 



In this condition, the larva swims 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 as a 

 Nemertid provided with the characteristic proboscis, and the 

 other organs of that group of Turbellaria. 



Many Trematoda, and all Taenioid Scolecida, again, present 

 an essentially similar process of internal gemmation, in virtue 



