OOV ASTACOCROTON, A NEW TYPE OF ACAEID, 



alive for a long time and eventually undergoes more or less shrivelling. The same 

 holds good of other reagents used cold, except Carnoy's solution, the fixing action 

 of which, however, was not satisfactory. Killing with hot water, followed by 

 treatment with various re-agents — Flemming without acetic, Henning, picrosul- 

 phuric, Hermann, — did not yield good results. On the whole the best sections so 

 far have been obtained with siDecimens treated with hot sublimate-acetic. Double 

 embedding (with either toluol-alcohol or alcohol-ether celloidin and paraffin) was 

 found to be of advantage. Staining was most satisfactorily effected by means of 

 haematoxylin (iron-alum method, Heidenhain, Delafield or Ehrlich) followed by 

 erythrosin. 



Most of the material available was collected by myself and fixed with hoi 

 sublimate-acetic. Though this was for the most part at least ten years old, it 

 was in a good state of preservation. More recently, since I became engaged in 

 this research, I have received highly valued assistance in the form of supplies of 

 live crayfishes from Prof. H. G. Chapman and Mr. F. A. McNeill. 



For comparison I have used species of Tetranychiis and Trombidium, and 

 various Hydrachnids, Tyroglyphids and Oribatids. 



I am indebted to the University of Sydney for a grant from the McCaughey 

 Research Fund which has defrayed the expenses incurred. My drawings have 

 been re-drawn for the purpose of reproduction by Mr. F. W. Atkins of the 

 Sydney Technical High School. 



2. Diagnosis of Astaeocroton. 



Adult female imago permanently parasitic on gills of Crayfish, eyeless, in- 

 capahle of swimming, devoid of tracheae. Integument of body thin, transparent, 

 devoid of ehitinous plates and practically hairless. Body swollen, ovoid, with 

 the legs displaced forwards so as to be all in front of the middle of the trunk. 

 Capitulum not gTeatly produced, with the mouth at the anterior end of its ventral 

 surface. Chelicerae with the second joint piercing, barbed. Pedipalpi power- 

 ful, the last joint provided with hooked spines, the penultimate not produced. 

 Epimera subequal, all distinct, except that the third and fourth are united for a 

 very short distance at their inner ends ; on the fused part but opposite the fourth, 

 is the aperture of a gland or group of glands — coxal or integumentary. The 

 legs are devoid of swimming hairs; each is armed terminally with a pair of 

 strongly hooked tridentate claws. The genital aperture is a longitudinal slit 

 close to the posterior extremity of the body. A little distance behind and above 

 it is the excretory aperture. The animal is oviparous; a large number of ripe 

 eggs, each enclosed in a thick shell, collect in the uterus, but their active develop- 

 ment does not begin till after they have been discharged. 



I propose to name the only species as yet known .4. molle. 



3. General Features. 



'Tlie permanently attached females reach a maximum length of about 2 mm. 

 When detached, full-grown specimens are able to climb about among the branchiae, 

 but, when set free in water, they are unable to swim, though making energetic 

 efforts to do so. Small specimens make more or less rapid progress through 

 the water. The general shape is oval, with a slight ventral flattening. The 

 integument is very thin, colourless and transparent, so that on the ventral side the 

 ova and other internal structures are clearly visible. In some specimens the 

 most conspicuous structures on this surface are a pair of rounded bodies, the 

 ■'coxal gland.s" (PI. xxxvi., fig. 1, ex.), situated between the bases of the fourth 



