gl6 Comparative Animal Physiology 



contraction can be elicited over either \)f two pathways from the mantle, 

 via the brain mass (traversing the stellate ganglion for both afferent and ef- 

 ferent fibers), or via the stellate ganglion alone.^^^- ^''" 



Molluscs, in general, have well developed local reflex centers which may 

 show some interaction but which have relatively restricted control of par- 

 ticular sensory and motor areas. In this respect molluscs are even more high 

 ly specialized than the segmental arthropods. 



CEPHALIC DOMINANCE IN INVERTEBRATES 



A step in complexity of nervous function beyond local or segmental re- 

 flexes is the increasing importance of centers near the anterior end of the 

 animal. This importance depends first on the concentration of sense organs 

 at the front end of the animal and second on coordinative functions of the 

 "brain" ganglia. 



Flatworms. In the free-livins flatworms the brain is associated with eves 

 and other sense organs. In the polyclad Thysanozoon the brain is needed for 

 spontaneous locornotion,^^" and after removal of the brain or head the poly- 

 clad Yungia is inactive and less sensitive than it is normally but it can be 

 stimulated mechanically or chemically to perform swimming movements.-'^"' 

 In Yungia, some hours after the operation, spontaneous and coordinated but 

 undirected movement occurs. In some marine polyclads swimming stops but 

 ciliary action persists if the brain is removed or split;^^** the rippling move- 

 ment (ataxic locomotion) continues, but extension, placing, and release (di- 

 taxic locomotion) and peristalsis stop since these require conduction from 

 the brain back over lateral cords. Righting appears to require the brain in 

 the polyclad Planocera but not in the triclad Planaria.^*^'^ In Planaria, loco- 

 motion and reflex extension of the pharynx and swallowing continue after 

 removal of the head, but normal behavior to food reappears only after the 

 brain regenerates.^*' 



In cestodes, a wave of contraction progresses backward across a cut through 

 lateral cords, being conducted reflexly by tension applied behind the cut.'^'**' 

 Cutting one lateral cord results in circus movements in those species of ne- 

 merteans in which transverse connections between lateral cords are incom- 

 plete, but not where the cords are connected. ^•''^ In summary, the flatworm 

 brain relays sensory messages and coordinates motor responses, there being 

 no other ganglia; its directive function is, therefore, very important. 



Annelids. An earthworm has sensory cells of various types— tactile, chem- 

 ical, and light receptors, scattered throughout the epidermis, but most con- 

 centrated in the prostomium and anterior segments. Only the sense cells 

 which are sensitive to dryness seem restricted to the prostomium.-^^'" Nereis 

 (Neanthes'), on the other hand, has fewer epidermal receptors; its effective 

 chemoreceptors are on palps and tentacles, and instead of scattered photo- 

 sensitive cells it has several pairs of eyes on its head. After the brain is re- 

 moved from an earthworm the anterior segments are lifted upward, it crawls 

 normally, appears restless and active, it can right itself, can copulate, eat, 

 and burrow^ in half an hour as compared with a normal time of one to two 

 minutes.-**-^' '•'•-^ After the brain is removed from a nereid worm, it no longer 

 feeds, does not burrow, is over-active and has lost its light sensitivity and 

 most of its chemical sensitivity.-*"'' -•^•' When the subesophageal ganglion is 



