December, '14] TOWER: LIFE HISTORY OF PROSPALTELLA 423 



Development of the Egg 



The egg is usually found lying about medially in the scale and slightly 

 nearer the pygidium end of the scale than the head end, although it 

 may be found almost anywhere in the body. Here the egg swells in 

 embryonic development to two to three times its size when laid and 

 the young larva may be seen forming within it. The larva is practi- 

 cally fully developed before hatching and lies curled within the egg. 

 On hatching it straightens itself out in the egg and the chorion, which 

 has become very thin, is ruptured, allowing the larva to escape into 

 the body of its host. 



First Larval Stage 



The larva, when hatched, is large compared with its host. Its 

 mouth-parts, which were fully formed while in the egg, now become 

 fully chitinized and the mandibles, which are the most prominent parts, 

 are very sharp, strongly chitinized and decidedly hook-like. 



The tracheal system, which lies close to the body surface and which 

 was only partially filled with air when the larva was in the egg, now 

 becomes filled and the two longitudinal main trunks, which lie on 

 either side of the body meeting anterior^ and posteriorly, forming an 

 oval, each show ten short stub-like branches. 



Apparently there are thirteen body segments, the thirteenth or 

 posterior segment lying partly within the basal portion of the tail. A 

 distinct head region containing the mouth-parts is marked off. This 

 region in the newly hatched larva is of the same diameter as the body, 

 but subsequent growth enlarges the body, while the diameter of the 

 head increases little if any. Thus the head region becomes more 

 definitely marked off. 



The tail is about one-fifth the length of the larva. It is seen in the 

 embryonic larva lying close to the body, while in the hatched larva it 

 is extended and is used for propulsion inside the host. A number of 

 pointed folds or hyaline scales, which protrude slightly from the sur- 

 face and point backward, are placed irregularly on the surface of the 

 tail and these doubtless make the tail an even more efficient organ of 

 propulsion. 



Oxygen must be obtained through the skin or from the food eaten, 

 as the larva lives submerged in a liquid medium having no connec- 

 tion with the tracheal system of its host or with the exterior, also its 

 own tracheal system does not connect with the outside of its own 

 body. 



Feeding is carried on by means of a sucking pharynx aided by the 

 mandibles. The ventral surface of the pharynx is a rigid chitinous 



