102 PROCEEDINGS OF THE ACADEMY OF [Feb., 



cephalad to the hypodermis, piercing the brain on their way, mean- 

 while dividing into minute branches which pass between the hypoder- 

 mal cells and attach themselves to the cuticle as described above. By 

 far the greater number of the fibres, however, cross in the midline, 

 forming a sort of chiasma, the fibres of one side passing over to the 

 opposite side, where they pierce the lateral lobe of the brain on that 

 side and, breaking up into fibrillse, are attached to the cuticle of the 

 antero-lateral aspect of the head, near the cells belonging to the first 

 preoral ciliated ring (fig. 19, v.l.m. 2). The dorsal longitudinal muscles, 

 on reaching the level of the mouth, bend downward and run directly 

 forward, to penetrate the lateral portion of the brain of the same side. 

 Here they meet the fibres of the ventro-lateral muscle of the opposite 

 side, and accompany them to their attachment. 



The muscles traversing the head cavity are four in number. The 

 first of these is composed of but a few fibres, which pass in a dorso- 

 ventral direction through the head cavity, just behin^d the brain (fig. 

 23, d.v.m.). The second traverses this space from side to side, while 

 the third and fourth form a pair which cross it obliquely. All of these 

 exhibit the usual brushlike subdivision of the ends of their fibres, 

 and the attachment to the cuticle already described. 



Muscles of the simple type described above have been found in all 

 the species of Dinophilus whose finer anatomy has been carefully 

 studied. Korschelt (1882) first found them in D. apatris, where they 

 form two layers, one of circular and one of longitudinal muscles, the 

 latter having the greater development. The muscles of D. gigas 

 (Weldon, 1886) comprise delicate transverse muscles on the ventral 

 side, and a bundle of fibres on each side of the trunk, running through 

 its whole length, just above the lateral nerve cords. Weldon believed 

 that the fibres of the circular muscles were continuous with processes 

 of certain of the hypodermal cells. Harmer (lS89a) mentions, in his 

 paper on D. tceniatus, longitudinal muscles which are similar to those 

 of D. gigas. Schimkewitsch (1895) studied the muscles of living 

 examples of the White Sea Dinophilus {D. vorticoides) with the 

 aid of methylin blue. Three layers of muscles were found in con- 

 tact with the body wall: an outer circular layer, whose fibres have a 

 regular metameric arrangement; a layer of oblic|ue fibres, and a layer 

 of longitudinal fibres, part of which are gathered together to form a 

 pair of ventral longitudinal muscles. The latter run through the 

 entire length of the trunk ; at its anterior end some of them are attached 

 to the body wall on the fold which separates the head from the trunk, 

 while others pass into the head, cross, and penetrate the brain, as 

 described for D. conklini. 



