Mr. A. Tulk on the Anatomy of Phalangium Opilio. 163 



segments (ds)j ten in all, are much better defined than the rest, 

 which increase in width towards the middle of the abdomen and 

 diminish again towards its anal end, where they curve down- 

 wards, following the convex form of the body ; so that the last of 

 the dorsal arcs (PI. III. fig. 2. o) presents itself as a very small 

 oval plate, wedged in betwixt the one which precedes it [ds) and 

 the last of the ventral segments, upon a level nearly with the 

 surface of which it lies. The transverse depressions which sepa- 

 rate the ventral segments from each other are deeper than those 

 upon the upper surface of the abdomen, and are rendered still 

 more distinct by the presence of rows of dark elongated puncta 

 {pa). The limit of the sternal portion of the thorax (PI. III. 

 fig. 2. st), which appears merely as a prolongation of the anterior 

 arcs, is effected by the first of the above transverse grooves {p), 

 terminated at either end by a large and impressed punctum. 

 Behind this the ventral arcs, five (5) in number, decrease in size 

 from before backwards, and may be very clearly demonstrated 

 by allowing the body of a harvest spider to dry, when the integu- 

 ment contracts into corresponding folds. Internally there are 

 slight ridges. The sternum [st], obtusely triangular in form, 

 extends as far forwards as the coxae of the third pair of legs, where 

 it forms a thickened, emarginate lip, which covers the aperture 

 leading to the sexual organs. Upon either side, and at a little 

 distance from the base of the steraal plate, is a deep groove (*), 

 situated between the posterior coxae and the under surface of the 

 abdomen, within which are the respiratory openings, and which 

 is boimded internally by a longitudinal ridge. 



The organs of locomotion^ eight in number, constitute, from 

 their extreme length, one of the most striking and characteristic 

 features in the outward form of the Phalangia. They radiate 

 from the under surface of the thorax, as in the Scorpionida and 

 other Arachnida, and are there attached by short, conical and 

 obtuse coxal joints (PI. III. fig. 2. c), the apices of which converge 

 inwards towards each other, while their opposite ends, tnincated, 

 form an articular cavity, and project upwards, upon either side of 

 the cephalo-thoracic plate. These joints are formed by simple 

 eflections of the integument, and are attached throughout nearly 

 their entire length to the thorax, so as to be immoveable, and thus 

 afford a fixed point of attachment for the muscles. Upon the in- 

 ternal side of the under surface of the thorax four projecting 

 septa are perceived, formed, with the exception of the most ante- 

 rior and posterior, by the contiguous sides of a pair of coxal joints, 

 the margins of which are connected together by a thinner and 

 more membranous portion of integument, which is readily thrown 

 into folds and again extended. The posterior septum is incom- 

 plete, being only formed internally by the side of the sternum, 

 and the inner extremity of the last coxal joint. The concavities 



