EMBRYONIC FORMS. 253 



onic forms in determining their position, as we noticed in regard 

 to the young trepang, starfish, worm, or molluscan ; for example, 

 one of the lowest fossil forms of the group to whJch the Zoo- 

 phytic Trepang belongs, a Hemic.o smites (p. 128, fig. 66) has 

 been mistaken for one of the Mollusca, a Chelyosoma, whose 

 body is covered by similar, many-sided, pavement-like scales. 

 Again, certain of the group of Articulata, for instance, some of 

 the Planarians, (like fig. 47, p. 92,) e. g 1 ., Eolidiceros, have a re- 

 markable resemblance in form to certain of the so-called naked 

 Mollusca, namely, Janus, and also exhibit the same habits, in the 

 manner of walking, and also in crawling back-downwards along 

 the surface of the water. It is not a little curious, too, that Eoli- 

 diceros and Janus should resemble each other in regard to their 

 nervous system.* 



Among Mollusca, the Bryozoa, especially those which have a 

 simple circle of tentacles around the mouth, have been mistaken 

 for Polyps. This point I have already discussed, when compar- 

 ing the Bryozoa with Polyps, in a previous lecture (p. 247). 



Even among Vertebrates, there are those, for instance, certain 

 kinds of Latnprey-eels, (Myxine,) which might at first sight be 

 mistaken for worms ; and in fact they have been described 

 as such by Linnaeus; and others, such as the Lancelet, (Amphi- 

 oxus,) (fig. 133, p. 226,) have been wrongly classified with sea- 

 slug's, a group of naked Mollusca. This was owing to the un- 

 developed state of the vertebral axis of the Lancelet, and the 

 otherwise singular resemblance of this fish to certain Mollusca 

 (Firolidce and the like) which swim rapidly in the open sea. 



* We owe to Blanchard (Voyage en Sicile, vol. in.) the first decided and 

 conclusive proof that the Planarians are true Articulates, by his demonstration 

 of the successive ganglionic repetitions along the nervous threads, at the right 

 and left sides of the mid-line of the body of a large Planarian (Polycladus Gayi. 

 Blanch.). 



