446 ACARINA. 



has attained this stage the original egg-shell is split into two valves and 

 eventually cast off, but the embryo remains enclosed within the cuticular 

 membrane shed at the first ecdysis. This cuticular membrane is spoken of 

 by Claparede as the deutovum. In the deutovum the embryo undergoes 

 further changes ; the chelicerse and pedipalpi coalesce and form the 

 proboscis ; a spacious body cavity with blood corpuscles appears ; and the 

 alimentary canal enclosing the yolk is formed. 



The larva now begins to move, the cuticular membrane enclosing it is 

 ruptured, and the larva becomes free. It does not long remain active, but 

 soon bores its way into the gills of its host, undergoes a fresh moult, and 

 becomes quiescent. The cuticular membrane of the moult just effected 

 swells up by the absorption of water and becomes spherical. Peculiar 

 changes take place in the tissues, and the limbs become, as in Myobia, 

 nearly absorbed, remaining however as small knobs. The larva swims 

 about as a spherical body within its shell. The feet next grow out afresh, 

 and the posterior pair is added. From the proboscis the palpi (of the 

 pedipalpi) grow out below. The larva again becomes free, and amongst 

 other changes the chelicerje grow out from the proboscis. A further ecdysis, 

 with a period of quiescence, intervenes between this second larval form and 

 the adult state. 



The changes in the appendages which appear common to the Mites 

 generally are (i) the late development of the fourth pair of appendages, which 

 results in the constant occurrence of an hexapodous larva ; and (2) the early 

 fusion of the chelicerae and pedipalpi to form a proboscis in which no trace 

 of the original appendages can be discerned. In most instances palpi and 

 stilets of variable form are subsequently developed in connexion with the 

 proboscis, and, as indicated in the above descriptions, are assumed to cor- 

 respond with the two original embryonic appendages. 



The history of the germinal layers. 



It is a somewhat remarkable fact that each of the groups of 

 the Arachnida so far studied has a different form of segmenta- 

 tion. The types of Chelifer and the Spiders are simple modi- 

 fications of the centrolecithal type, while that of Scorpio, though 

 apparently meroblastic, is probably to be regarded in the same 

 light (vide p. 120 and p. 434). The early development begins in 

 the Scorpion and Spiders with the formation of a ventral plate, 

 and there can be but little doubt that Chelifer is provided 

 with an homologous structure, though very probably modified, 

 owing to the small amount of food-yolk and early period of 

 hatching. 



The history of the layers and their conversion into the organs 

 has been studied in the case of the Scorpion (Metschnikoff, No. 



