334 ASTACOCROTOX, A NEW TYPE OP ACAItID, 



The pharyux (PL xxxvi., figs. 8-10, ph; PL xxxvii., figs. 11-12) has the 

 general structure usual in the Acarida, corresponding closely in most respects 

 with that of Trombidium as described by Henking and that of Uiyas as described 

 by Michael. An important difference, however, is that the swallowing muscles 

 ("Schluckmuskeln" of Henking, "transverse muscles" of Michael) which are so 

 conspicuous in transverse sections of this region in Trombidium and occur also 

 in the Hydrachnids, or at least in some of them, are here entirely absent — the 

 sucking muscles being opposed merely by the elasticity of the wall of the pharynx. 

 The chief agents in the sucking action are the "dilatores pharyngis" muscles of 

 Michael (1895, p. 182). 'I'o the result of the contraction of these muscles in 

 enlarging the lumen of the pharynx, a contribution is made by short muscular 

 bundles arising from the inner surface of the ventral cuticle in the middle line 

 and inserted into the ventral half -tube of the pharynx, mainly into a keel -like 

 process which projects downwards from the latter; this muscle which may be 

 termed depressor pharyngis, is figured by Micliael in Thijas (1895, fig. 23), but is 

 not lettered and is not mentioned in the text. 



The "Giftdriisen" which are also very conspicuous in transverse sections of 

 the capitulum of Trombidium and are described and figured in that genus by 

 Henking (1882) are entirely absent in Astacocroton. So also is the azygous 

 gland referred to by Michael (1895, p. 192) as lying (in Thyas) between the 

 two sets of muscles which tilt up the chelicerae. 



There is, however, in this region, a gland (?) not represented in T-rombidium 

 and not recorded, so far as I can ascertain, as occurring in any other Aearid. 

 This, which I propose to name pharyngeal gland, is rather (if it be indeed glan- 

 dular) of the nature of a group of large unicellular than of a pair of compound 

 glands. These (PI. xxxvii., fig. 12, g) are situated between and around the dilator 

 muscles of the pharynx. Each is a somewhat pyramidal cell, 0.1 mm. in length, 

 with a rounded base directed upwards, and the attenuated apex becoming lost 

 among the fibres of the dilator muscles near the dorsal wall of the pharynx. Each 

 cell has a nucleus about .01 mm. in diameter, and vacuolated cytoplasm whi-ch is 

 Jiot readily affected by stains. 



The oesophagus is extremely narrow, with an excessively minute lumen. As 

 usual it perforates the central nerve-mass to open into the mesenteron. The 

 latter consists of an anterior median sac extending across the whole breadth of 

 the body immediately behind the dorsal salivary gland, and a pair of caeca 

 which extend back to the posterior end of the body. As in the Prostigmata in 

 general, there is no hind-g-ut or anus. 'I'he two caeca (PL xxxvii., figs. 14-16, 

 ent) are separated from one another by the median excretory organ (ex) and 

 occupy with it all the dorsal part of the cavity of the body — the ventral part 

 being taken up by the ovary and the uterus. The median sac and the caeca are 

 of essentially the same structure. A basement membrane supports an epithelium 

 of an extremely irregular character. The contents are invariably the blood of 

 the crayfish, disseminated through which are usually to be seen numbers of the 

 characteristic blood-corpuscles. 



The epithelium resembles the corresponding layer in the Acarida in general 

 as described by Michael (1895, 1894-97), Thor''(19b4) and others. Some^of the 

 cells are relatively small, others are produced into the lumen and dilated distaJly, 

 the proximal part usually becoming constricted. The protoplasm of the large 

 cells and, to some extent, of the small also, is loaded with a variety of meta- 

 plasmic bodies, in the form of granules, spherules and irregular concretions, which 

 vary very greatly in character and relative abundance in different individuals. 



