34 COMPARATIVE ANATOMY. 



median pedal sinus even though the foot l)e absent. 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 by 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 richly ciliated epithe- 

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

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



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, and the nerves supplying the mantle-cavity complex arise from 

 a more restricted section, but in the arrangement of the brain, pedal and lateral 

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

 blance between the two orders. In every case the brain, 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 buccal wall and the adjacent 

 regions of the body. About the bases of the atrial cirri these fibres connect 

 with accumulations of nerve cells which may be 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 number in Limifossor, 

 are connected with the brain by several nerves, and on the other hand 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 unite immediately before plunging into the accessory gangli- 

 onic masses attached to the brain but retain a delicate connective-tissue sheath, 



