220 DR. A. A. W. HUBRECHT. 



E. Circulatory and respiratory apparatus. — In all the genera 

 of Amphineura a heart, situated dorsally, close to the posterior 

 extremity of the body, a median dorsal and a median ventral 

 blood-vessel, are the principal parts of the circulatory apparatus. 

 Paired auricles to the heart are present in the Chitons ; their 

 presence in Neomenia and Proneomenia is not yet put beyond 

 all doubt, but still rendered very probable. The dbrsal vessel 

 is the direct anterior continuation of the heart. The latter is 

 situated in a cavity in which blood is never found, which may best 

 be compared to the body-cavity, and to which the inappropriate 

 name of pericardium has been given. It is closed on all sides, 

 with the only exceptions hereafter (p. 228) to be mentioned. 

 The longitudinal vessels open out anteriorly amongst the tissues, 

 the circulation being lacunar for a very great portion (around 

 the intestinal folds/", ex). A part of the lacunar circulation in 

 the foot of the Chitons will most probably have to be regarded 

 as the equivalent of the ventral blood-vessel of the Solenogastres, 

 which similarly lies below the horizontal muscular diaphragm. 

 For details about the circulatory apparatus of Chiton, reference 

 may be given to Middendorff's researches (IG) ; suffice it to say 

 that here, as in the Solenogastres, the blood is pumped by the 

 heart out of the gills and driven forwards along the median 

 dorsal vessel towards the genital gland and the head. 



As to the respiratory apparatus very different degrees of de- 

 velopment are present amongst the Amphineura. In Proneo- 

 menia special branchiae were vainly sought for, and if a tuft of 

 hollow threads in one of the folds of the pharynx must not be 

 looked upon as such — functionally at least — we are forced to the 

 conclusion that respiration takes place all along the wall of the 

 intestine and the foot, and perhaps more especially in the rectum. 



Both in Neomenia and Chatoderma retractile branchiae have 

 been demonstrated at the posterior extremity of the body. They 

 are tuft-like in the former (11), distinctly paired in the latter 

 genus (6), where the anal opening lies between them. 



In Chitonelliis they are no longer paired, but are numerous 

 and stretch between the foot and mantle, to the right and left 

 of the anus, about as far as half way along the body, each 

 branchial process having to be looked upon as a unit in com- 

 parison to the gills of the Prosobranchia (2, 20). In the genus 

 Chiton the lateral branchial series are even extended further 

 forwards, reaching as far as the head. Hand in hand with this 

 marches a complication in the circulatory apparatus. 



F. Excretory and generative organs. — This apparatus and its 

 different modifications in the various genera and species of the 

 Amphineura perhaps requires more tlian any other renewed and 

 careful investigation. A few years ago the confusion was even 



