180 Dr. T. Williams on the Mechanism of Aquatic are contiguous, and the anterior eyes of the four intermediate ones forming the trapezoid, which are near to each other, are the smallest and darkest of the eight. The legs are provided with hairs, and have a yellowish brown hue ; the anterior and posterior pairs, which are the longest, are equal in length, and the third pair is the shortest ; each tarsus is terminated by three claws ; the two superior ones are curved and slightly pectinated, and the inferior one is inflected near its base. The palpi resemble the legs in colour. The abdomen is oviform, hairy, glossy, convex above, and projects over the base of the cephalo-thorax; it is of ^ pale brown colour, with obscure spots of a deeper shade, the under part being rather the darkest; the sexual organs have a reddish brown hue, and their anterior margin is prominent and semicircular. The colours of the sexes are similar. The male has the humeral joint of its palpi curved towards the cephalo-thorax, which has a narrow^ indentation immediately behind each lateral pair of eyes; the radial is larger than the cubital joint and pro-jects a strong obtuse apophysis from its extremity, in front, towards the inner side ; the digital joint is somewhat oval, with a large lobe on the outer side ; it is convex and hairy externally, concave within, comprising the palpal organs, which are highly developed, complicated in structure, with a curved prominent process at their base, on the outer side, another, situated under-neath, which has its extremity enlarged and depressed, and two long, contiguous, filiform, black spines, originating near their base, on the outer side, which pass obliquely downwards, and curving round their extremity, extend considerably beyond the termination of the digital joint : the colour of these organs -is red-brown. Early in October 1853 both sexes of Neriene herhigrada, in S mature state, were detected among coarse herbage and moss growing in woods on the northern slope of Gallt y Rhyg. Like Neriene sulcata, this species makes a near approximation to the spiders of the genus Walckenaera. XVllI. — On the Mechanism of Aquatic Respiration and on the Structure of the Organs of Breathing in Invertebrate Animals^ By Thomas Williams, M.D. Lond., Licentiate of the Royal College of Physicians, formerly Demonstrator on Structural Anatomy at Guy^s Hospital, and now of Swansea. [With two Plate.] {Continued from p. 137.] The orbit of the blood-proper in the Annelid is conducted in obedience to the simplest hydraulic principles. The Annelid is
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