166 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



(Fig. 35). The lappets are fringed with long cilia, and be- 

 tween them, where the head would fit into a helmet, is the 

 aperture of a mouth, which leads into a cascal pouch-like ali- 

 mentary cavity. This larva was named by Miiller, who dis- 

 covered it, Pllldium gyrans. On each side of the ventral 

 face of the PlUdimn^ two involutions of the integument take 

 place. Aggregations of cells in relation with these, and 

 probably forming part of the mesoblast, appear, eventually in- 

 close the alimentary canal of the Pilidium^ and give rise to 

 an elongated vermiform body, in which the characteristic feat- 

 ures of a Nemertean soon become discernible (Fig. 36). The 

 worm thus developed becomes detached (Fig. 37) and falls to 

 the bottom, carrying with it the alimentary canal of the Pi- 

 lidium, and leaving the ciliated integument to perish. 



In this remarkable process of development the formation 

 of the Nemertean body may be compared, on the one hand, 

 to that of the segmented mesoblast in Annelida and Arthro- 

 poda^ and, on the other, to that of an Echinoderm (especially 

 Echinus)^ within its larva. 



The Rotifera. — The " wheel-animalcules," as they were 

 termed by the older observers, on account of the appearance 

 of rotation produced, as in many Annelid larvae, by the w^ork- 

 ing of the vibratile cilia w^th which the oral end of the body 

 is provided, were formerly included among the Infusoria. 

 However, they are true Metazoa^ as their vitellus undergoes 

 division into blastomeres, and the tissues of the body are pro- 

 duced by the metamorphosis of the cells into w^hich the blas- 

 tomeres are converted. They are free or adherent, but never 

 absolutely fixed animals, and they do not multiply by gem- 

 mation or fission. The oral end of the body is usually broader 

 than the opposite extremity, and presents the form of a disk, 

 sometimes produced into tentacle-like prolongations (Fig. 39). 

 The edges of this trochal disk are fringed with long cilia, but 

 the general surface of the body, instead of being ciliated, 

 as in the Turhellaria^ is formed by a dense, generally chiti- 

 nous, cuticular layer, which is sometimes converted into a kind 

 of shell and variously sculptured. Transverse constrictions, 

 which are slight in the anterior part of the body, but may 

 become more marked toward its posterior end, give rise to an 

 imperfect segmentation. The segments do not appear to ex- 

 ceed six, and the divisions are less marked in the tubicolous 

 than in the free Potifera. The mouth is a funnel-shaped 

 cavity, situated in the middle, or on one side, of the trochal 



