346 J. M. D. OLMSTED 



the longitudinal muscles the diameter of the animal may be 

 twice what it is when those muscles are relaxed ; the contraction 

 of the longitudinal muscles of one side may produce a curving of 

 the body to that side, either throughout its entire length, or 

 more locally, according to the extent to which the contraction 

 affects the muscles of that side; the contraction of all the cir- 

 cular muscles may produce an extreme elongation of the animal ; 

 or, finally, local contraction of circular muscles may produce 

 annular constrictions (a very common condition of all Synaptidae) 

 at irregular intervals. 



Stimulation of the very posterior end often causes, in addi- 

 tion to local constriction, a bending away from the source of 

 stimulation, a reaction in which the longitudinal muscles along 

 one side of the body come into play; or the body may be slightly 

 shortened, in which all the longitudinal muscles of the posterior 

 half of the animal are equally contracted. The former reaction is 

 much the more common. 



But the most striking reaction which Synaptula exhibits is the 

 sudden withdrawal of all the tentacles into the mouth region. 

 This occurs when a moderate stimulus is applied to any of the 

 regions at the anterior end of the animal, e.g., the tentacles, the 

 mouth disc, or the area which extends from the base of the ten- 

 tacles backward 2 or 3 mm. This sudden turning 'outside in' 

 is the result of very severe contraction of the circular muscles 

 of the head region followed by that of the longitudinal muscles. 

 The contraction of the circular muscles of the head region natur- 

 ally drives the body fluid backward. When the longitudinal 

 muscles, which are attached to the calcareous ring just below 

 the tentacles, strongly contract they draw the ring, together with 

 the whole head region, back into the body, until the pressure of 

 the body fluid prevents further involution. The posterior ends 

 of animals which have been decapitated present a plump appear- 

 ance, since the circular muscles for a distance of some 3 mm. back 

 of the cut remain strongly contracted, both closing the wound 

 opening, and forcing the body fluid into the posterior end. A 

 withdrawal of the tentacles is seldom shown unless the anterior 

 end is stimulated. 



