382 
Tetrarhynclms erinaceus 
fibres, while a similar fusion takes place with respect to the ventral fibres. 
(Fig. 4, the lower figure.) They now form the anterior section of the 
general longitudinal musculature of the strobila. 
(3) Intrinsic proboscidial muscles. The proboscis bulbs are the 
posterior terminal parts of the proboscidial apparatus, and are situated 
near the posterior part of the scolex at the place where the latter 
exhibits a distinct swelling. They are represented in PI. XIX, fig. 3 as 
they are seen in cleared preparations of the scolex, and they are also 
represented in section in PL XIX, fig. 4. Each of them is a cylinder which 
is about 2 mm. in length and about 0'7 mm. in diameter. The bulb is 
nearly circular in transverse section and consists of an outer wall of 
cuticular nature which is continuous with the wall of the sheath and 
with that of the proboscis; of an inner wall, also continuous with 
the sheath; and of an intermediate layer of muscle fibres. This 
muscular part takes up b}' far the greater part of the thickness of the 
wall of the bulb: it is not of uniform thickness and fails entirely at the 
part of the wall nearest to the integument of the scolex and just here 
the cuticular outer part of the wall of the bulb is greatly thickened and 
forms the place of origin and insertion of the muscle fibres composing 
the intermediate layer. The diameter of the lumen of the bulb is only 
about one-third of the total thickness of the latter, and the lumen itself 
is not quite circular in transverse section. 
The muscle fibres included in this intermediate part of the bulb 
wall are an-anged in two series which run obliquely round, being inserted 
at either end into the thickened cuticular external wall. They con¬ 
tinually decussate with each other crossing at an angle of about 45°. 
They are not arranged in bundles or laminae but lie quite loosely from 
each other except at certain places where the fibres appear to be twisted 
round each other. Obviously the effect of the contraction of these 
muscles will be to reduce the diameter of the bulbs and so to exert 
pressure on the fluid contained in the lumina of the sheaths. 
(4) Tlie Retractor muscle of the proboscis. This structure (see PI. 
XIX, fig. 5) is a compact bundle of smooth fibres each of which is about 
2 /r in diameter, the whole bundle measuring about 0'03 mm. in total 
thickness. It originates at the extreme posterior end of the proboscis 
bulb and is inserted into the internal wall of the proboscis at the 
extreme tip of the latter. The muscles run axially through the sheaths 
and bulbs and are quite free from the latter except at their ends. They 
must be capable of contraction to about one-third of their entire lengths. 
They function of course in the invagination of the proboscides. 
