MUSCULAR SYSTEM 465 



tails irresistibly take our thoughts back to those of Annelid 

 worms. There is also what may be a vestige of an ancient 

 excretory system in a pair of brown funnels, which extend be- 

 tween body cavity and atrial cavity, opposite the posterior end 

 of the pharynx. 



Lancelets are very muscular animals, the proportion of muscle 

 to the rest of the body being unusually great. As we have 

 noticed, the muscles are arranged in segments, a primitive con- 

 dition seen in all vertebrate embryos, and visible throughout 

 life in most fishes. The muscle-segments or myomeres are like 

 widely open V's with the apices directed forwards. They are 

 separated by partitions of connective tissue to which the 

 numerous longitudinally disposed and flattened muscle-fibres 

 (that compose each segment) are attached both in front and be- 

 hind. In this, and in the dove-tailing, and in the fact that the 

 muscle-segments of the two sides alternate, we see adaptations 

 to energetic movement. The body is bent from side to side, 

 as in fishes, the muscles of one side contracting while those of 

 the opposite side are relaxed. 



" The locomotion of Amphioxus is a rapid, curiously irregular 

 wriggle, often accompanied by somersault-like movements " 

 (Parker, 1908). It can swim head first or tail first. When the 

 tail is stimulated, the animal moves head first ; when the head 

 or anterior region is stimulated, it moves tail first. Although 

 its movements are energetic, they cannot be kept up for more 

 than a few moments ; the animal soon drops down and lies 

 motionless, and it becomes unresponsive if it is excited re- 

 peatedly within a short time. 



Lancelets are characteristically burrowing animals. When 

 they drop on to the sandy bottom they may disappear with a 

 sudden spring, or they may lie for a time straightened out, and 

 then arching their bodies disappear beneath the surface. They 

 often lie with the mouth agape protruding on the surface of the 

 sand. They usually enter the sand tail foremost, but some- 

 times the head goes in first. While the free individuals are 

 always straight, those buried may have " a very tortuous outline, 

 as though they had crowded their way in between the coarse 

 pieces of shell and coral " (Parker, 1908). 



The Lancelet is a segmented animal, as is plainly seen in 

 the musculature, and as can be seen in the way the nerves 



30 



