34 COMPARATIVE ANATOMY. 



modiaii pedal sinus oven thoujih (he loot bo abspiit. In the posterior end of 

 the body the blood is collected in a branchial sinus, if gills be present, or is con- 

 veyed to the posterior end of the heart by means of clearly defined channels 

 in the neighborhood of the rectum. In the Neomeniina the haemoglobin is 

 contained in the corpuscles, and in the Chaetodermatina liy the plasma. 



In the Chaetodermatina two plume like respiratory organs, which spring 

 from the anterior wall of the cloacal chamber, are in all essential respects like 

 those of the Chitons and are doubtless true ctenidia, the space wherein they are 

 contained being the mantle cavity. In the Neomeniina such organs are absent, 

 though the cloacal wall may be developed into folds in some species of large 

 size, penetrated by numerous sinuses and covered with a i-ichly ciliated ejiithe- 

 liiim. It is believed by some investigators that such lamellae are incipient or 

 degenerate ctenidia but there is little to support such a tiieory. 



Apart from these organs it is probable that respiration takes place over the 

 surface of the body, especially along the ventral furrow if such be present. It 

 has been suggested also that the great buccal folds may possess a combined 

 respiratory and sensory function. 



Nervous System. — In the Solenogastres there is a more pronounced con- 

 centration of the nerve cells to form definite ganglionic enlargements than in 

 the Chitons, anil the nerves supjilying the mantle-cavity complex arise from 

 a more restricted section, l)ut in the arrangement of the brain, ]iedal and lateral 

 ganglia, and the labio-buccal system there is a very distinct fundamental resem- 

 blance between tlie two orders. In every case the lirain, usually if not always 

 bilobed, is situated on the dorsal side of the alimentary canal about the inter- 

 section of the mouth and pharynx. From its anterior face three pairs of nerves 

 originate in the Neomeniina and innervate the l^uccal wall and the adjacent 

 regions of the body. About the bases of the atrial cii-ri these fibres connect 

 with accumulations of nerve cells which may l)e the homologue of the great 

 ganglionic masses in contact with the brain in the Chaetodermatina. In this 

 last named group these accessory nerve masses, ten in numbei- in Limifossoi-, 

 are connected with the lii'ain by several nerves, and on the other hantl give rise 

 to fibres which innervate the sensory buccal plate (Mundschild). In what 

 appears to be the most primitive condition three pairs of connectives, the pedal, 

 lateral, and labio-buccal, take their separate origin from the brain as in the 

 Chitons. Such a state of affairs is the rule in the Neomeniina. In Chaetoderma 

 erudita these cords imite innnediately before plunging into the accessory gangli- 

 onic masses attaclied to the l)i'ain l)ut retain a delicate connective-tissue sheath. 



