OF THE ACAEID^ TO THE ARACHNIDA. 291 



Kramer. — " Zur Naturgeschichte der Milben." Arch. f. 



Naturgesch. 1876. 

 Kramer. — " Die Seginentirung bei den Milben." Arch. f. 



Naturgesch. 1882. 

 Michael, A. D. — British Oribatidae. Eay Society, 1884. 

 Pagenstecher. — Beitrage zur Auatomie der Milben, Tlieile 



i.-ii. Leipzig, 1860-61. 

 "Winkler — " Das Herz der Acarinen &c." Arb. Zool. In- 



stituts Wien, 1888. 

 Winkler. — "Anatomie der Gamasiden." Tom. cit. 



EXPLANATION OF PLATE XX 



N.B. — In all the drawings the line across the figure indicates the line of 

 demarcation between thorax and abdomen. 



Fig. 1. Outline of Tetranychns tiliarum, showing the three segmental furrows 

 with arrangement of dorsal musculature ; the dots show the position 

 of the bristles. 1. The mandibles fused in the middle line and 

 somewhat telescoped into the body. 2. The pedipalps also fused 

 in the middle line. 3, 4, 6, 6. The four pairs of ambulatory limbs 

 belonging to the four thoracic segments. I, II, III. The three abdo- 

 minal segments, with the genital and anal apertures. 



Fig. 2. A. Diagrammatic segmentation of an Araneid. 1-6. The six segments 

 of the cephalothorax, each provided with a pair of limbs. I-X. 

 The segments of the abdomen, showing the positions of the genital 

 and anal apertures. 

 B. Segmentation of Tetranychus (cf. fig. 1) for comparison with A. 

 1-6. The cephalothoracic segments homologous with 1-6 in A. 

 I, II, III. The abdominal segments, of which I and II are homologous 

 with I and II in A, and III homologous with X, showing that 

 seven segments are undeveloped. 



Fig. 3. A. Diagram of heart of an Araneid lying entirely in the abdomen. 



B. Diagram of 3-legged larva of Gamasus fucorum (adapted from 

 Winkler), showing position and stage of development of the heart 

 for comparison with A. 



Fig. 4 (from Lang's ' Text-book of Comparative Anatomy '). A. Alimentary 

 canal of an Araneid {Mygale) : m, Malpighian tubule ; rv, rectal 

 Tcsicle. B. The same of Gamasus. A comparison of the two shows 

 that it is the abdominal portion of the adult Araneid alimentary 

 canal which is not developed in the Mite. 



Fig. 5. A. Ventral ganglion of an Araneid {Mygale, adapted from Lang's ' Text- 

 book of Comparative Anatomy '). 2-6. The five posterior cephalo- 

 thoracic appendages, each supplied with a nerve, a, a. The visceral 

 nerves radiating from an abdominal ganglion. 

 B. The same of Gamasus (after Winkler). 2-6. Nerves branching to 

 2nd to 6th appendages, a, a. The ventral nerves branching directly 

 from the ventral ganglion. 



LINN. JOUEN. — ZOOLOGY, VOL. XXIT. 23 



