336 SUEGEOX r. H. WKLCH on the ANAXOiiy 01 



circular muscle passes downwards aud terminates in a tendon 

 (fig. 13) which is continued ou to the bulb (figs. 14 & 16). The 

 action of these layers is apparent from the direction of the fibres ; 

 the outer cylindrical layer evolves the proboscis ; the circular layer 

 gives it stability aud firmnesss ; the thick circular basal muscle 

 retracts it. These muscular layers are made up of solid cylindrical 

 fibrillse arranged parallel to each other and of an exquisite delicate 

 yet decided texture ; moreover there are transverse markings upon 

 them which are not sufliciently regular to be pronounced stride, 

 yet unquestionably they closely approximate voluntary vertebrate 

 muscular tissue*. The sheath of the proboscis is kept in situ by 

 muscular bands and fibrous tissue which diverge from it into the 

 surrounding head-substance ; and at the point where it contracts 

 with the tendon of the muscle (fig. 13, d), there the fibres are 

 very strongly pronounced. The retractor muscle is -~- inch in 

 length by --^ inch in diameter; and its tendon passes down for 

 a short distance, and merges into the muscular layers of the 

 bulb. 



These bulbs are four in number, one to each proboscis ; they 

 are elongated ovoidal structures, in shape markedly resembling a 

 sausage, y-^-y inch in length by yj o inch in diameter ; they are 

 arranged side by side within the expansion of the fibrous visceral 

 boundary of the colony, passing through the neck to merge in 

 the cuticular layer of the free end of the head (fig. 8, d ; fig. 16, d). 

 In structure they are composed of planes of muscle w^hose fibres 

 cross each other in all directions ; these planes form an outer 

 sti-atum covered externally by a prolongation of the sheath of the 

 proboscis ; and within this outer stratum are enclosed the special 

 fibres connected with the retractor tendon (fig. 14). These spe- 

 cial fibres are seen to pass on one side of the bulb (fig. 14, a, h) 

 to its bottom and then to curve upwards on the other side to the 

 top, near where they enter, apparently there merging in the 

 fibres of the outer stratum. That this intricate arrangement of 

 muscular bands is connected with the retraction of the proboscis 

 cannot be questioned, and would appear to be dependent on the 

 shifting position of the central tendon in the exserted or retracted 

 state of the proboscis ; for in the process of inversion (accom- 

 plished by the upper fibres of the central tendon, retractor 



* The recent observations of Mr. Schaefer on the structure of muscular fibre 

 (voluntary) seem to me to throw light on this p'oint and also receive light from 

 these details. 



