OF OECANTIIUS AND TELEAS. 269 



structures varies greatly in its relative position with the degree of the development 

 of the larva. It forms the border of a deep groove which indicates the anterior 

 limit of the abdominal region. From this groove the abdominal wall gradually swells out 

 into a flask-shaped body, which carries in front of its equator a pair of lateral flattened 

 expansions of the cuticula, filled t with hypodermic cells. These bodies are the fin-pads- 

 On their dorsal surface one finds numerous small dentate papillae, while from their margins 

 and outer portions are given off from 15 to oO long, curved, unjointed, hollow, colorless 

 bristles. These fin-pads are moved in a plane at right angles to the dorso-ventral axis of 

 the body by several well developed muscles. The tip of the abdomen, which is not termi- 

 nal in position but lies on the ventral surface of the body, is continued into a long, hollow, 

 cylindrical appendage, usually terminating in a dentate knob (pi. 24, fig. 18), the teeth 

 of which are on its ventral side. When the larva is quiet the terminal end of the caudal 

 appendage lies opposite the mouth, and when the larva feeds it serves both to break up 

 the yolk and to shove it into the spoon-shaped lower lip. 



When the larva is about to enter upon an ecdysis the cell structures become dark from 

 .... the presence of granules in the protoplasm, the hypodermis shrinks away from 



: ; w<3- ^ ie cu ticula, the walls of the mesenteron shorten and thicken, the muscles 

 ©0 e€>- lose their striate appearance, the protoplasmic contents of the antennae, man- 

 Fig. 41 Sur- dibles, and caudal appendage recede from their sheaths and together with the 

 tiie e iiypoae'r- hypodermic cells of the fin-pads are drawn into the body mass. The cuticula 

 "h'frri °lrv* e now swells and loses its definite shape and ultimately becomes a thin sac 

 x 125, loosely enveloping the larva. The cells of the hypodermis next secrete a 



new cuticula and the larva at once assumes its former activity, rupturing the cast off 

 cuticula and burying itself again in the yolk matter of its host. 



While the spindle-shaped larva is changing into the mandibulate form, the cell elements 

 of the body lose their distinctive characters so entirely that the slightly differentiated 

 organs are scarcely to be distinguished in the cell mass. As soon as a cuticula is secreted, 

 however, the relations of the cell groups become distinct and now for the first time the 

 muscular fibres of the abdomen are to be seen. There is, then, a histolysis of the cell groups 

 but no cell-fusion or cytolysis. The muscular system of the second larval stage consists of 

 thirteen paired and two median unpaired muscles. The largest muscles of the body are 

 the mandibular adductors, while their corresponding abductors are the smallest ones pres- 

 ent in the body at this time. These four muscles which almost fill the cavity of the 

 cephalo-thorax are pyramidal in form and have their origin in the dorsal w r all of the ceph- 

 alo-thorax and their insertion into the upper and lower rami of the mandible of the right 

 and left sides respectively. When fully extended flie mandibles project at right angles 

 from the body, but when completely retracted they lie in close contact with the body wall, 

 their tips fitting into the lower lip. There are two pairs of dorsal muscles connecting the 

 head and thorax. The more superficial extends lengthwise the body (pi. 24, fig. 22) while 

 the deeper pair extends somewhat obliquely to this axis. The large abductor of the fin 

 extends at right angles to the longitudinal axis of the body, following the dorsal half of 

 the equator of the abdomen to its origin in the hypodermis of either side. Two other fin 

 muscles extend obliquely from this organ backward toward the median line, in the hypo- 

 dermis of which they take their origin. The insertion of all the fin-muscles is in the 



