ARCHI ANNELIDA. 451 



deposited in the water, with but little more protection than is 

 afforded by the vitelline membrane. 



In the former case the development is direct and there is no 

 larval stage, the development taking place at the expense of 

 albuminous matter, which is included in the cocoon. But in the 

 latter case the young undergo only a small part of their develop- 

 ment in the egg-membranes, and are hatched out as larvae at 

 an early stage, to undergo the rest of their development in 

 the free-swimming state. The larval form, most commonly, one 

 may almost say invariably, found, is that called the trochosphere, 

 This larval form, which we have already met with in Mollusca, 

 is fully described further on. The larva of Polygordius, which 

 is known as the larva of Loven, is a typical trochosphere (Fig. 360), 

 and the larvae of many Chaetopoda, and of Eehiuroidea (Fig. 431), 

 are typical trochospheres. 



The Annelida are classified as follows : 



CLASS 1. ARCHIANNELIDA. 

 ,, 2. CHAETOPODA. 



Order 1. Polychaeta. 

 ,, 2. Oligochaeta. 

 CLASS 3. HIRUDINEA. 



4. ECHIUROIDEA (GEPHYREA ARMATA). 



For reasons fully stated in the sequel, we have thought it 

 necessary to break up the old group Gephyrea, and to exclude 

 some of its divisions (Sipunculvidea, Priapuloidea, Phoronis) from 

 the Annelida. The Eehiuroidea, however, we retain as being 

 obviously true Annelids. 



Class I. ARCHIANNELIDA.* 



Marine Chaetopoda without setae or parapodia. 



There are two genera in this order, Polygordius Schn. and Proto- 

 drilus "Hatschek, the former found in sand of European seas, and 

 the latter in the sand of an inland sea-lake at Faro near Messina. 

 They are elongated and thread-like, and are entirely without setae 

 or parapodia. The head has two tentacles and two ciliated pits (Fig. 

 358). The segmentation is but faintly marked externally, by slight 



* Fraipont, " Le Genre Polygordius," Fauna and Flora d. Golfes v. Neapel 

 xiv., 1887. Hatschek, ' Protodrilus," Arb. Zool. Inst. Wien, 3, 1881. Weldon, 

 "Dinophilus Gigas," Q. J. M. S., 1886, 27. Harmer, " Dinophilus," Journ. 

 Mar. Biol. Ass., New Series, 1, 1889. For a description of Polygordius see 

 T. J. Parker, Lessons in Elementary Biology, London, 1891. 



