CH^TODERMA, NEOMENIA, AXD CHITON. 5 71 



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

 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 Cha^toderina, with the allied genus 

 J^eometiia^ and the Chitons, especially in the arrangement of 

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

 the three into a group of Amphineura — thus separating the 

 Chitons from the 3Iollusca altogether. 



Section 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 Pedicellina 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 which 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 present a structure as simple as that 



1 " Anatomie des Chcetoderma nitidulumy {Zeitsc/irift fio' iviss. Zoologie^ 

 1870.) 



2 " Vergleichende Anatomie des Nerv en systems der Mollusken," 1877. 



3 " Untersv;chun.2:en zur Erforscliuug der genealogischen Grundlage des 

 Crustaceensystem«," 1876. 



* " Beitr^ge znr Kenntniss der Bryozoen." {ZeiUclirift fhr wiss. Zoologie^ 

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



