NOTES AND MEMORANDA. 415 



Each stigma leads into a backwardly directed tube, which soon 

 divides into two branches. Of these, the inner and smaller is directed 

 forwards, and passes immediately into a trachea : while the outer 

 and larger passes obliquely outwards, backwards, and upwards, and 

 becomes continuous, in like manner, with a large trachea. 



The tubes which put the trachete in communication with the 

 stigmata diifer from the stigmatic pouches of Jidus, in that the latter 

 are directed forwards from the external aperture. The tubes are 

 probably metamorphosed portions of tracheal stems. 



Structure of the Hydraehnida. — Croneberg gives * a brief 

 account of his observations, which were originally published in 

 Eussian, In all species which he has examined he has found a 

 chitinous framework internal to the labium, which consists of two 

 pieces which bend forwards and unite above the mouth ; these form a 

 chitinous groove which is connected with a system of muscles, all of 

 which go to make up a powerful suctorial apparatus. Above the 

 mouth there are two other chitinous ridges, which enclose the com- 

 mencement of the two primary trunks of the tracheal system. 



The oesophagus passes through the ganglion and then widens out 

 into a large stomach ; it is provided with a number of cfecal sacs, 

 which vary in number from five to thirty -four ; these are all connected 

 together and are invested by an epithelium, which consists of large 

 brown cells, and represents the liver. The fatty body seems to be 

 represented by a layer of smaller and more transparent cells, covering 

 the stomach and the excretory organs. These latter, in all cases, end 

 by a portion which passes directly to the anus, which persists even 

 where the rectum is absent, and the midgut ends blindly. 



The buccal glands are arranged in three groups, of which two are 

 racemose and one tubular ; tliey all three have a common duct. The 

 generative glands vary greatly in character and position ; in Eijlais 

 they consist of a system of communicating longitudinal and transverse 

 canals, which form a network around the stomach ; there is a single 

 orifice. In Nescea and Hydrachna the ovaries are circular in form, but 

 the testes consist of five large pyriform tubes united at their base 

 (^Nescea), or of a number of smaller and pedunculated saccules. In all 

 three cases the seminal duct passes into a muscular bulb, and the 

 oviducts into a wide, muscular vagina. 



Acarina found parasitic in the Cellular Tissue and Air-sacs of 

 Birds. — To the general rule that the superficial or cutaneous parasites 

 of animals— the so-called Epizoa — belong to the group of the Articu- 

 lata, and that the internal parasites are worms, there are, as might be 

 expected, some not inconsiderable exceptions ; thus there are Filarice 

 which infest the skin, and there are insects [(Estrida) to be found in 

 the stomach of the horse, the pharynx of the deer, and in the cranial 

 sinuses of the sheep. M. Megnin, in writing on this subject,! makes 

 but brief reference to the Linguatididce, but these Arachnoids, as they 

 are ordinarily considered, may be referred to in a little more detail as 



* ' Zool. Anzeiger,' i. (1878) 316. 



t ' Journ. Anat. et Phya.' (Robin), xv. (1879) 123. 



