BIOLOGY OF LIMNERIUM VALIDUM. 85 



ently not on the sides. Each fold on the venter represents a body 

 segment, though some may be more or less double; the folds on the 

 dorsum, however, are plainly double, as two occur on each segment. 

 The body is thus found to be 12-segmented, excluding the tail 

 appendage Avhich is plainly a ventral outgrowth of the last segment. 



The appendage itself sometimes appears to be distinctly ringed, 

 due undoubtedly to the folding of the integument. It is slender, 

 tapering, and about four-fifths as long as the body. During em- 

 bryonic development it is bent sharply forward and appressed to the 

 venter; aft«r eclosion it is bent to the axis of the body for a short 

 period at an angle of about 90°, though later in life it extends 

 straight backward. Its color and that of the body is transparent 

 whitish. 



The total length of the newly hatched larva is about 0.64 mm.; 

 without tail, only 0.41 mm.; the width of the head is 0.11 mm. and 

 that of the thorax 0.10 mm. 



FUNCTION OF THE TAIL APPENDAGE. 



In case of larva? observed innnediately after hatching, the tracheal 

 system could be made out easily, and Avas filled with air without 

 doubt, though 

 necessarily of 

 the closed or 

 a p n e u s t i c 

 type. Only 

 one fine trach- 



e "1 1 b r 'I n <^ h ^^^'^- ^6. — Llmncrium vaJtdum: First-stage larva of large size, show- 

 ing silk glands and nervous system. Enlarged 50 times. (Original.) 



could be cUs- 



tinguished in the tail, and it was clearly not important enough to 

 indicate that the tail is a tracheal gill. The function of the tail, 

 however, is probably res])iratory, and the organ might properly be 

 termed a blood gill. There is nothing in its structure to contradict 

 this view, as it is a simple, hollow tube lined Ayith hypodennal cells, 

 and undoubtedly filled wuth blood a gi-eater part of the time. Since 

 the larva lies free in the body cavity of the host it is constantly 

 bathed in blood and lymj^h fluids, from Avhich the oxygen of its own 

 blood must be derived through the delicate integument of the tail, 

 or other parts of the body, especially while still small. 



APPEARANCE OF OLDER LARV.E. 



The larva makes its escape from the ^gg, by bursting open the cho- 

 rion irregularly at the anterior pole (fig. 34), possibly with the aid 

 olthe mandibles. After hatching it develojjs rapidly if located in a 

 suitable host, and molts for the first time in probably about T to 10 



