138 



DR. T. DAVIDSON ON EECENT BEACHIOPODA. 



Diagrammatic view of 

 the muscles of Argiope 

 (after A. E. Shipley). 



" The muscles [p. 504] of Av^\o\)q {ClsteJla) consist of four pairs ; of these two pairs are 

 connected with tlie movement of tlie shell upon the stalk, tlie other two with closing and 

 opening the shell. Each of these last is composed of two parts 

 and is usually considered to rej)resent two distinct muscles, thus '"' 



making the number six Of the two pairs of adjusters, which 



are both inserted into the peduncle, one pair arises from each shell 

 (A'alve) (woodcut, fig. 14, d.ad and v.acl). The pair arising from the 

 dorsal shell has its origin opposite the commencement of the intes- 

 tine, and tlie muscles of this pair are close to each other, one on each 

 side of the median line ; they pass down and are inserted by a tendi- 

 nous cord which passes directly into the substance of the stalk. The 

 adjusters of the other shell have their point of origin a little nearer 

 the posterior border of the shell, and are rather larger, they also are 

 inserted into the stalk in a similar manner. The principal function 

 of tliese muscles is without doubt to raise and lower the animal upon 



its stalk The occlusor muscles [p. 505] have two points of origin 



upon each side of the m.edian line of the dorsal shell, one in front of 

 the other. They correspond with the two parts of the muscle which 

 were termed by Hancock the occlusor anterior and posterior. Their origin is external to 

 that of the adjustor muscles, and their anterior limit passes far into the two anterior lateral 

 divisions of the body cavity formed by the septum (woodcut, fig. 14, oc.m). The muscles 

 of each side which are at first quite distinct soon unite into a common tendon which takes 

 an obliquely downward direction, passing close vmder the central nervous system; and here 

 it is connected with the homogeneous substance supporting the chief ganglion on each 

 side of the oesophagus, 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 tendons. The division into two parts, of 

 the divaricators, is much less marked than is the ease 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 

 (woodcut, fig. 14, d.m). The axis upon which the shell turns joasses 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 [Cistella) all the muscles are unstriated. 



" The peduncle of Argiope [p. 506] appears to be an unusually large development of the 

 homogeneous supporting substance which occurs so frequently in the body of Brachio- 



poda The ovaries of Argiope p. 507] are of a bright brick red color, resembling 



the red rays on the shell of Argiope cuneata Each ovary [p. 508] appears to be 



foi'med of a membrane continuous with the body-wall When the eggs are ripe they 



drop off into the body cavity, the capsule apparently bursting. In the body cavity they 

 are taken by the inner end of the oviduct and thus pass into the brood pouch." 



