Pomona College, Claremont, California 81 



expiiudiiuj. The fibers in this muscle, instead of running from the 

 origin to the insertion of the muscle as a whole run dorsoventrally 

 from wall to wall so that any stimulus acting on the nerve which 

 supplies this muscle and causing the fibers to contract would cause 

 the muscle to become rigid, of less diameter and of greater length. 

 Since the origin of tiie muscle is in the back part of the pharnyx and 

 since this organ is bound to the body wall by connective tissue and 

 muscles, the "expansion" of the mandibidar muscle must force the 

 mandible forward and with it the forceps teeth of the maxilhe, which 

 are closely bound to it. This action extrudes the teeth and a sec- 

 ondary reflex seems to be established that causes them to be snapped 

 together forcibly soon after they are extruded. This reaction is 

 carried on with such \igor that I ha\'e known eunicids to bite them- 

 selves into two or three pieces while dying in fixatives. 



The advantage of this extrusion system over the more primitive 

 proboscoideal eversion found in the forms without the denticular 

 pouch can be seen in the quickness of the reaction, its superior \igor 

 anil the fact that the teeth are extruded first rather than as a final 

 consequence ot the comparatively slow eversion of a soft fleshy 

 proboscis. 



SUMMARY 



1. The annelid, Arcicodn seinimcictilata, is a highly specialized 

 and evolved member of the lumbriconereidae, inhabiting the mussel 

 beds near Laguna. 



2. Reconstructions were made stereographically as described 

 in this paper. 



3. The brain is \'ery complex and highly specialized sensorially. 

 It is di\'ided into the sensory, connecti\'e, nuchal and visceral sys- 

 tems, is symmetrical and has a central tubal cavity running through 

 the lower part of the main cross-connective portion of the brain, 

 from front to back, parallel to a cavity which is partly filled with 

 muscles and glands, and runs between the \-isceral ganglion and 

 the main brain and forward between the dorsal and \entral lobes of 

 the fore brain. This central cavity, taken together with the very 

 complex and convoluted olifactory forebrain presents an appear- 

 ance which seems quite similar to that described by Patten in I/nniiliis 

 and other invertebrates, but which can probably be best explained 



