SKELETON MUSCULAR AND NERVOUS SYSTEMS. 413 



body cavity, or, according to another view, are spaces 

 secondarily hollowed out in the tissues, and Q pseudocalic. 



Skeleton. 



This is very slightly developed, for there is not only no 

 bone, but the supporting material is not even definitely 

 cartilaginous. 



(a) The notochord runs from tip to tip. It consists of 

 vacuolated cells, and probably owes its supporting power to 

 their turgidity (cf. plants]. 



(&) The pharynx is supported by a system of chitinoid 

 bars, which border the numerous gill slits. There is also a 

 paired longitudinal plate along the mid ventral groove of 

 the pharynx. 



(c) The mouth is embraced by two curved bars, each 

 segmented into about a dozen pieces, which bear filaments 

 supporting the cirri. 



(d) The sheath which envelops the notochord and is 

 continued round the nerve cord, the septa of connective 

 tissue which divide the muscle segments, and produce the 

 <-shaped markings ; the 250 " fin rays " which ^support the 

 dorsal and ventral fins, may also be included here. 



Muscular System. 



The swimming movements are caused by lateral wriggling 

 of the body. This is effected by the segmented lateral 

 muscles, in which the muscle fibres run longitudinally. On 

 the ventral surface between the mouth and the atriopore 

 there is a transverse set of fibres which help to drive out 

 the water from the atrial cavity. Other muscles occur in 

 the region of the mouth and elsewhere. Nearly all the 

 fibres are striated. 



Nervous System. 



The dorsal nerve cord is shorter than the notochord, and 

 has no anterior swelling. It gives origin to two sets of 

 nerves, dorsal and ventral. The dorsal nerves correspond 

 to the segments, except in the anterior region, where they 

 are more numerous, and the first five pairs may be regarded 

 as cerebral ; the ventral nerves are minute and numerous. 

 The two sets are compared to the single-rooted sensory 



