250 DISCOVERY REPORTS 



the mouth is formed at about one-third the distance from the anterior end of the 

 primitive gut. This stage has been figured by Harmer (1905, PI. XIII, fig. 173). Later 

 still, the pre-oral region of the primitive gut develops the characteristic appearance of 

 the notochord. From the earliest stage its homology is revealed by the position of the 

 heart and pericardium. 



To draw any general conclusion from the development of the bud may not be justi- 

 fiable, but it is recorded here since it shows a distinct stage in the formation of the noto- 

 chord, which may serve for comparison when the embryonic development is worked out. 



MUSCULAR SYSTEM 



The muscular system of Cepholodisais is well developed, and the zooids are capable 

 of vigorous contraction. The chief muscles can be grouped as follows: {a) the muscles 

 of the stalk, which are continuous with {b) the postero-ventral longitudinal muscles of 

 the trunk, {c) longitudinal muscles of the collar, {d) longitudinal muscle of the arms, 

 (e) transverse muscles of the proboscis and (/) "problematic" tissue situated on the 

 roof of the collar pores. 



Longitudinal muscles of the stolon are reflected on the ventral wall of the trunk. At 

 the junction of the stolon and the body these muscles form three distinct tracts, which, 

 diverging and broadening out slightly, run along the ventral body wall towards the 

 bases of the right and left collar pores. On their course forwards some of the fibres ter- 

 minate in the ventral body wall , so that the two bands become weaker as they approach the 

 collar pores and are finally inserted into the septum between the trunk and collar cavities. 



Two strong muscles are given off from the region of the collar canals ; these traverse 

 the collar cavities on each side of the mouth and end in the septum between the proboscis 

 and collar cavities. From their relation to the mouth they are called the oral muscles. 

 In the arms there are four bundles of longitudinal muscles, two strong bundles ventrally, 

 one on each side of the food groove, and two weaker bundles in the dorsal wall. 



The transverse muscles in the proboscis are grouped into bundles, which radiate from 

 the septum between the proboscis and collar cavities. These muscles are attached to the 

 ventral wall of the proboscis and their contraction enables the disc of the proboscis to be 

 used as a sucker for the creeping movement of the zooids when they come out of the ostia. 



Besides these definite muscle fibres there is a very prominent bundle of " problematic " 

 tissue situated on the roof of each of the collar canals. These were regarded as lateral 

 skeletal masses forming a fulcrum for the pre-oral muscles (Masterman, 1898 b, p. 348), 

 but Harmer (1905, p. 46) observed that they constitute a series of oval lamellae which 

 stain like muscle fibres, each connected along half its circumference with the roof of the 

 collar canal and with the external body wall, the other half being freely exposed in the 

 collar cavity. The oval lamellae appear in all details to be muscle fibres. They originate 

 from the roof of the collar pore and radiate towards the dorsal and ventral walls of the 

 collar. Most of these fibres are inserted into the collar septa, only those towards the 

 middle remaining free in the collar cavity. In C. densus the distal ends of these fibres 



