422 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



cate directly with the blood sinuses through the pericardium. 

 In many Pteropods and Heteropods they are rhythmically 

 contractile. 



As in the Lamellibrayichiata, so in many Odontophora^ 

 simple or branched canals traverse the substance of the foot 

 and open externally by a more or less conspicuous pore, which 

 is usually situated upon its inferior face. These aquiferous 

 canals, as they have been termed, appear, in many cases, to 

 open by their inner ends into the blood sinuses, and thus to 

 establish a direct communication between the blood and the 

 surroundiiig water. In species of Pyrula, Agassiz found that 

 colored fluids injected into the pore passed into and filled the 

 blood-vessels genera%. But it may be doubted whether 

 these canals should be regarded as a special system of ves- 

 sels, rather than as blood sinuses which open externally. 



The arrangement of the centres of the nervous system in 

 Dentalium ^ most nearly approaches that which exists in the 

 Jjaniellibranchiata. Two cerebral ganglia lie close together 

 on the hnemal side of the oesophagus. A long commissural 

 cord connects each of them with one of the pedal ganglia, 

 which are also closelj^ united. A second long commissure 

 passes backward from the cerebral ganglia, and often presents 

 a ganglionic enlargement at its origin. It unites with one of 

 two ganglia, situated close to the anus, and connected, in 

 front of it, by a rather long transverse commissure. The 

 nerves distributed to the posterior half of the mantle are 

 given off from these ganglia, and those to its middle region 

 from the anterior end of the commissure or its ganglionic en- 

 largement. There seems no reason to doubt that the ganglia 

 close to the anus, together with the ganglionic enlargements 

 at the anterior ends of the commissures which connect them 

 with the cerebral ganglia, correspond w^ith the parieto- 

 splanchnic ganglia of the Lamellib ranch s, and that the cere- 

 bral and pedal ganglia are the homologues of those so named 

 in the latter Mollusks. 



In addition to this approximation of part of the gangli- 

 onic mass of the parieto-splanchnic system to the cerebral 

 ganglia, Dentalium differs from the Lamellibranchs and re- 

 sembles other Odontophora, in the possession of a sj^stem of 

 buccal nerves, which arise from the cerebral ganglia, and in 

 which minute ganglia are developed. The nerves which pro- 



» See Lacaze-Duthiers, " Orijanisation du Dentale." 



