CHJETODERMA, NEOMENIA, AXD CHITON. 57I 



discovery of L. Graff/ that the minute spines of Chcetoderma 

 are calcified. It is a further peculiarity of this genus that 

 two distinct nerve-cords proceed from the cerebral ganglia 

 parallel with one another on each side of the body, in the 

 place of the single median nerve-cord of other members of the 

 group. 



Dr. Jhering ^ has directed attention to certain points of 

 resemblance between Chcetoderma, with the allied genus 

 JSfeomenia., and the Chitons, especially in the arrangement of 

 the trunks of the nervous system ; and he proposes to unite 

 the thiee into a group of AmpJimeura — thus separating the 

 Chitons from the Mollusca altogether. 



SECTioisr V. — I regret that I have been unable to make use 

 of Claus's recently-published important contributions to the 

 history of the development of the Crustacea.^ 



Section VI. — The thorough examination of the structure 

 of Pedicelllna and Loxosoma by Nitsche ^ has shown that 

 the differences between the ectoproctous and the endoproctous 

 Polyzoa are of a more fundamental character than had been 

 suspected. In the Ectoprocta, in fact, the endocyst consists 

 of two layers, an outer and an inner, of which the former is 

 the representative of the ectoderm in other animals. The lat- 

 ter lines the wall of the " perivisceral cavity," and is reflected 

 thence, like a peritoneal tunic, over the tentacular sheath and 

 into the interior of the tentacula, whence it is continued on to 

 the alimentary canal, of w4iich it forms the external invest- 

 ment. The endoderm, which lines the alimentary canal, is, 

 of course, continuous, through the oral opening, with the ec- 

 toderm. 



In the Endoprocta, on the contrary, the endocyst is com- 

 posed of only one layer, and the endoderm of the alimentary 

 canal has no second or external coat. The " perivisceral 

 cavity," or interspace between the endoderm and ectoderm, 

 is occupied by ramified mesodermal cells. 



Thus the Endoprocta i:>resent a structure as simple as that 



1 " Anatomie des Chmtodenna nitidulum.'''' {Z&itschrift fur wiss. Zooloffie, 

 isrr..) 



2 " Yerfflc'ichende Anatomie das Nervensystems der Mollusken," 1877. 



3 " Untersuchunsren znr Erforschmig der genealogischen Grundlage dos 

 Crustaeeensystem«," 187*>. 



•* " Beitnifre zur Kenntniss der Bryozoen." (ZeiUclrift fi'ir wiss. Zoologie, 

 1870 and 1875.) Compare Burrois (" Comptes Eendus," 1875). 



