282 APLACOPHOBA. 



it has a dorsal armor ot seven slightly imbricating plates, exactly 

 comparable with the seven-valved stage in the development of 

 Chiton. This observation definitely fixes the posi'tion of the Apla- 

 cophores, as a degraded group of Amphineura, which had its incep- 

 tion in a Chilon-like ancestor, and has undergone reduction of the 

 foot and dorsal armor by living in mud at depths below wave dis- 

 turbance. 



The simplification of the digestive tract in Aplacophores has 

 doubtless been a secondary modification, due, as Simroth holds, to 

 the adoption of a carnivorous diet ; but the posterior gills, reduced 

 to a single pair (for the numerous gill folds of Neomenia, etc., are 

 not true ctenidia), the spiculose integument and the nervous system, 

 are doubtless primitive structures inherited from polypi acophorous 

 ancestors. 



There is considerable diversity in habits and mode of life among 

 the forms now known. Chcetoderma, Neomenia, Proneomenia and 

 Ichthyomenia are free-living forms, lying imbedded in mud, head 

 downward, like a Dentalium, selecting their food of organic parti- 

 cles from the surrounding ooze. Rhopalomenia, Nematomenia, Lepi- 

 domenia, etc., are parasitic upon Hydroids, Gorgouians and Corals, 

 upon the branches of which they crawl and coil themselves. 



It is likely that future search will reveal Aplacophores in all 

 seas, those at present known being a mere fragment of the existing 

 fauna. 



I am informed by Prof. A. E. Verrill that no less than six species 

 of three or four genera, including Neomenia, occur in the Fish 

 Commission collections off the eastern U. S. They are still un- 

 published. 



The best general account of the anatomy of the group is that of 

 Simroth in Bronn's Klassen und Ordnungen des Thier-Reichs, Vol. 

 III. The memoirs of Wiren, Pruvot, Thiele, Kowalevski and 

 Marion and Hubrecht, are the most important special treatises upon 

 the subject. 



Development. 



Pruvot (Comptes Rendus cxi, p. 689-692, 1890) has observed the 

 development of Myzomenia banyulensis. A brief summary of his 

 observations is as follows : 



The eggs are globular, and laid singly, few at a time. Segmenta- 

 tion begins an hour after they are laid, proceeding rapidly, and 



