92 PHYSIOLOGICAL SERIES. 



end of the animal to the other ; these are intersected at 

 regular distances, corresponding to the breadth of the seg- 

 ments. 0. C. 58, 



Quatrefages, Ann. Sci. Nat., ser. 5, t. xi. 1869, p. 309. 



C. 18. Two specimens of a Leech (Ilirudo medicinalis) . The 

 upper specimen is a transverse section taken through the 

 middle of the body. The viscera have been removed to show 

 several muscular strands passing perpendicularly through 

 the body from the dorsal to the ventral wall, their con- 

 traction flattening the body for swimming. These muscles 

 lie between the gastric pouches within metamerically 

 disposed connective-tissue lamellae. The individual muscle- 

 fibres are branched at either end. The lower specimen 

 shows the longitudinal muscles which form the inner and 

 most powerful layer of the body-wall. They are specially 

 developed along the ventral mid-line. The muscle-fibres 

 are spindle-shaped cells composed o longitudinally fibril- 

 lated cortex of contractile substance, surrounding a central 

 core of undifferentiated protoplasm within which lies the 

 nucleus. The protoplasm of the core extends to the peri- 

 phery of the cell between the cortical fibrils. 0. C. 57 A. 

 Leuckart, Die Parasiten des Menschen, Bd. i. 1894, 

 p. 568. 



C. 19. A Leech (Hirudo medicinalis), having part o its external 

 tegument dissected off to show its longitudinal subcutaneous 

 muscles. 0. C. 57. Hunterian. 



C. 20. A Brachiopod ( Terebratella cruenta) from which the viscera 

 and a portion of the left side of the valves have been 

 removed to show the valvular and peduncular muscles. 

 The former concerned in opening and closing the valves 

 consist of two pairs of occlusors and a pair of divaricators. 

 The two occlusors of either side arise from the dorsal valve 

 slightly in front of the hinge ; for the first part of their 

 course they are independent and fleshy, but about the middle 

 of the body they assume a tendinous appearance, unite with 

 one another, and are finally inserted by a common expanded 



