Fig. 202. Four successive stages in the development of Myobia musculi.
(After Claparède.)
s1-s4. post-oral segments; ch. cheliceræ; pd. pedipalpi; pr proboscis formed by the coalescence of the cheliceræ and pedipalpi; p1, p2, etc. ambulatory appendages.
In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are not accompanied by the peculiar changes observable in Myobia: the cheliceræ and pedipalpi fuse however to form the proboscis. The first larval form is hexapodous, and the last pair of appendages is formed at a subsequent ecdysis.
In Atax Bonzi, a form parasitic on Unio, the development and metamorphosis 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 in the usual fashion. They undergo considerable differentiation; the cheliceræ and pedipalpi approaching each other at the anterior extremity of the body, and the three ambulatory legs becoming segmented and clawed. An œsophagus, a stomach, and an œsophageal 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 Claparède as the deutovum. In the deutovum the embryo undergoes further changes; the cheliceræ 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 cheliceræ 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 cheliceræ 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 correspond 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 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. [434]), and of the Spiders; and a close agreement has been found to obtain between them.
It will be convenient to take the latter group as type, and simply to call attention to any points in which the two groups differ.