TRACHEATA. 369 



boscis are formed the rudimentary palpi of the second pair of appendages, 

 and two elongated needles representing the chelicerse. 



In the cheese mite (Tyroglyphus) the embryo has two ecdyses which 

 are not accompanied by the peculiar changes observable in Myobia : the 

 chelicerse and pedipalpi fuse however to form the proboscis. The first 

 larval form is hexapodous, and the last pair of appendages is formed at 

 a subseqiient ecdysis. 



In A tax Bonzi, a form parasitic on Unio, the development and meta- 

 morphosis are even more complicated than in Myobia. The first ecdysis 

 occurs before the formation of the limbs, and shortly after the ventral 

 plate has become divided into segments. Within the cuticular membrane 

 resulting from the first ecdysis the anterior five pairs of limbs spring 

 out iu the usual fashion. They undergo considerable differentiation ; the 

 ehelicerje and pedipalpi approaching each other at the anterior extremity 

 of the body, and the three ambulatory legs becoming segmented and 

 clawed. An oesophagus, a stomach, and an oesophageal nerve-ring are also 

 formed. When the larva 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 deutovuui. In 

 the deutovuni 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 iu 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 cheliceras 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 (1) 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 chelicerte 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 connection 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 segmentation. 

 The types of Chelifer and the Spiders are simple modifications 



B. E. 24 



