256 GENERAL CONSIDERATIONS. 



be a true mantle cavity, though at present there is no more actual proof for 

 such a view than for the one which considers it to be an anal space. In my 

 opinion this fold, lying in the mid-ventral hne and supphed with two glands, 

 holding essentially the same position as the glands and creeping surface in the 

 3'oung Chiton, is a true foot, the homologue of the Chiton foot, and has been 

 derived from a conunon ancestor. In the Chaetodermatidae there are few 

 traces of its existence; in the Neomeniina it varies from an exceedingly small 

 organ to one relatively wide and comprising several folds, but so far as I can 

 judge these are secondary features having to do solely with modifications within 

 the group. 



That the ventral nerve cords cannot be considered pedal ganglia because 

 they innervate the ventral side of the body as well as the ventral fold, and in 

 this respect are unUke other moUuscs, appears to me inconclusive evidence. 

 The supraoesophageal gangUa in both molluscs and annelids are probably 

 derived from homologous groups of cells of the head vesicle, and there is a strong 

 probabiHty that the anterior pair of ventral or suboesophageal ganglia in the 

 armeUds is the counterpart of the pedal gangUa of molluscs, the repetition of 

 the ventral gangUa in the annehd being correlated with metameric segmenta- 

 tion. The ventral ganglia of annelids innervate the ventral suface, the entire 

 body wall in fact, but with the development of the mantle and its associated 

 complex in the molluscs a new set of ganglia, the palUal, appeared w^hich inner- 

 vate these typically molluscan organs. Whether this theoretical view is accepted 

 or not it is certainly true that the ventral surface of the body of the molluscan 

 ancestor, before a definite creeping surface became differentiated, was innervated 

 and continues to be innervated whether the foot includes the entire ventral side 

 of the body or not. \Miere the foot is small, as in the modern Solenogastre, 

 and the body wall continues to form a portion of the under surface both continue 

 to be supphed by this ventral set of nerves. WTiere the foot constitutes the 

 entire ventral side of the body it alone is so supplied. 



The broader features of the nervous sj-stem have been described in the 

 Pacific report, and the study of the various species in the present collection 

 merely confirms the general behef that in the Solenogastres it is reducible to 

 one fundamental plan. In every case the brain is attached to three connectives, 

 the lateral, pedal, and labiobuccal, and anteriorly develops nerves which pass 

 into ganglionic masses (precerebral ganglia) in close proximity to the brain 

 (Chaetodermatina) or attached to the bases of the atrial cirri (Noemeniina). 

 The lateral and pedal cords course to the posterior end of the body where they 



