ANATOMY, PHYSIOLOGY. 
Arachn. 7 
the Arthrogastra. The branchiae have been lost, but no special organs of 
respiration have yet been acquired. 
Winkler (97) treats of the heart in various groups of Arachnida. 
This organ was observed in numerous Gamasidce. It is a short, broad 
organ, situated in the anterior portion of the abdomen; on the upper 
surface are two valvular openings; the aorta opens freely iuto the body- 
cavity above the cerebral ganglion ; behind and in front the heart is con- 
nected with the dorsal surface by fibres. A similar organ was noticed in 
nymphs and young females of Ixodes ricinus. In the Phalangidce, the 
heart lies in the anterior abdominal region, and is provided with two 
pairs of lateral clefts. In Chernetidce, the heart has not been previously 
observed. In young forms of Obisium silvaticum it was seen to be situ- 
ated in the three first abdominal somites, and to be provided posteriorly 
with a pair of clefts. As in the Phalangidce, a valvular fold separates 
the heart from the aorta. The arterial system of Scorpions, Houssay 
(39) maintains, consists of two groups of vessels — dorsal and ventral — 
which are connected with each other at the anterior end of the body ; 
the connecting vessels envelope the nerve-cords which run from the 
cerebral to the thoracic mass. The resemblance between the arrange- 
ment in the Scorpion and in Limulus is pointed out. 
Saint-Remy (78) states that the brain in Tegenaria domestica, Epeira 
diademata , and Phalangium opilio is formed on the same plan as in 
Scorpion. There are two sets of nerve cells, those with a layer of proto- 
plasm round the nucleus, and those without this layer ; these latter exist 
only in the brain, and are especially abundant in the optic portions of it. 
In Scorpion the ganglion for the pectines resembles structurally the 
antennary ganglion, hence it seems probable that the former appendages 
are sense-organs. The Araneina and Phalangida possess nothing like 
this ganglion. 
Bruce (15) also describes the structure of the brain. It is divided 
into an anterior and posterior portion by an ingrowth of investing peri- 
neurium ; the former innervates the rostrum, the lateral projecting 
portions of which resemble the antennae of Insects ; the latter supplies 
tho labrum, which, as in Insects, is a pairod structure. The mandibular 
ganglion is situated below this posterior portion. 
For remarks upon the presence of a heart in Gamasus, see Kramer 
(58) and Michael (69). 
Bruce (15) describes a sense-organ in the coxal- joint of the fourth 
pair of legs in a young Phrynus. The same author points out the 
resemblances between this Arachnid and Limulus. 
For the comparative anatomy of the Arachnida, see Nuhn (72). 
Bertkau (10) has studied the eyes of Spiders. 
Physiology. 
Dahl (26) has investigated the sensory powers of various Spiders. 
Attus arenatus was influenced by loud knocking; this species and Xysticus 
lanio could very imperfectly distinguish colours, but this faculty existed 
in Epeira cornuta and in Drassus quadripunctatus. A male Epeira pata- 
