Ou the Stmcture and Development of Argiope. 505 



bable, as far as Argiope is concerned. The occlusor muscles have two 

 points of orig-iu upon each side of the median line of the dorsal shell, 

 oue in front of the other. They correspond with the two parts of the 

 musele which were termed by Hancock the occlusor anterior and pos- 

 terior. Their origin is exterual to that of the adjustor muscles, and 

 tbeir anterior limit passes far into the two anterior lateral divisions of 

 the body cavity formed by the septum (fig. 12 oc.m.]. The muscles of 

 each side which are at first quite distinct soon unite into a common ten- 

 don which takes an obliquely downward direction , passing close under 

 the central nervous system ; and here it is connected with the homo- 

 geueous substance supporting the chief ganglion oneach side of the Oeso- 

 phagus, and is inserted by an expanded extremity in the ventral shell just 

 to one side of the median ridge. The Insertion is a little anterior to the 

 posterior limit of the intestine which lies between the two tendous. The 

 division into two parts, of the divaricators, is much less marked than is 

 the case with the occlusors. These muscles arise, one upon each side 

 of the median line in the posterior part of the ventral shell , a little 

 behind the Insertion of the occlusors, they pass straight across and their 

 tendons are inserted into the most posterior part of the dorsal valve in 

 the middle line (fig. 12 d.m). The axis upon which the shell turns 

 passes through the teeth and sockets mentioned in the description of 

 the shell, and this lies in front of the Insertion of the divaricator muscles; 

 hence a contraction of these muscles has the effect of opening the shell. 

 In Argiope all the muscles are unstriated. The fibres are somewhat 

 flattened or oval in transverse section ; they are quite free from each 

 other ; there appears to be no interfibrillar substance : and they Stretch 

 the whole leugth from their origin tili they fuse with the tendon. A 

 transverse section through their point of origin shews that the fibres 

 arise in regulär rows , the long diameter of the section of each fibre in 

 one row making an obtuse angle with that of the neighbouriug fibre in 

 the next row. An oval nucleus lies upon some part of the length of each 

 fibre. At the point where the musele arises from the shell there seems 

 to be a special development of the homogeneous supporting substance 

 mentioned in the description of the lophophore. The musele fibres after 

 a louger or shorter course all pass into a tendon , which in the fresh 

 State is white and glistening. In section the tendons appear homo- 

 geneous or faintly striated in the longitudinal direction : they either end 

 m the stalk which is composed of a similar tissue, or in the homogeneous 

 supporting substance which is developed at some places in the body 

 wall, and which histologically very much resembles the tendon. 



