744 MR FRANK J. COLE 



so that there is no sharp transition between the simple and crossing portions of the 

 muscle. In the pre-abdominal portion of the obliquus, therefore, there is ventrally a 

 very curious interlacing, extending from immediately behind the mouth to the 

 branchial apertures, where it ceases, the fasciculi of one side having to pass under- 

 neath the fasciculi of the other side before emerging on to the surface of that side of 

 the body on which they are inserted. For example, a fasciculus, let us say, of the 

 right side arises dorso-laterally from the superficial external fascia. It courses obliquely 

 backwards and downwards, and leaves the fascia to pass internal to the slime sacks. 

 Median to the slime sacks it passes internally to the terminal portions of the fasciculi 

 of the left side (being separated from the fascia by these), emerges on to the surface 

 again at the mid-ventral line, and is now continued on the left side over the fasciculi 

 of the latter side, separating them from the superficial fascia, into which it is inserted. 

 Hence, in spite of the interdigitation, all the fasciculi arise and are inserted into the 

 superficial external fascia. The anatomy of this portion of the obliquus is illustrated 

 in fig. 10, which shows posteriorly two fasciculi on each side dissected clear of the 

 overlapping fasciculi, so as to expose the whole of their course. Anteriorly the over- 

 lapping portions are wider and closer knit, and the first few fasciculi on each side are 

 usually wider than the remainder. Omitting the very small posterior fasciculi, which 

 are difficult to delimit, there are about 100 on each side. 



Behind the heart (fig. 5) the obliquus covers superficially the ventral half or belly 

 surface of the body. Hence its dorsal abdominal border is roughly coincident with the 

 roof of the body cavity. Its fibres pass downwards and backwards in a slight curve, 

 and above the line of the slime sacks it covers externally the ventral portion of the 

 M. parietalis, whilst the slime sacks themselves in the relaxed condition lie in depres- 

 sions on its outer surface. Ventrally it covers externally the M. rectus, and between 

 the ventral border of the parietalis and the dorsal border of the rectus it forms the 

 only muscular wall of the abdominal cavity (fig. 1). Behind the cloaca it has no 

 appreciable existence.* The fibres of the superficial external fascia, which everywhere 

 covers the obliquus, course in a direction opposite to that of the fibres of the muscle. 



The function of the obliquus is to powerfully compress the walls of the body. In 

 the region of the body in front of the gill apertures this compression, owing to the 

 interdigitation of the fasciculi ventrally, is by far the most powerful. This is very 

 obvious when watching a living animal, when one sees how the branchial region of 

 the body is, as it were, wrung dry like a sponge. Another important function of the 

 obliquus is to express the pre-cloacal slime sacks, which have no intrinsic musculature.t 

 When the obliquus is relaxed the expanded slime sacks lie in depressions on its surface, 

 so that when the muscle contracts they are driven against the skin, which here closely 

 investing them and having little give, they are flattened between the obliquus and 

 the skin. 



* But cp. the description of the sphincter cloacae and the transversa? caudalis. 



t J. Muller states, however, that in Bdellostoma each slime sack is surrounded by a special muscular membrane. 



