ARACHNIDA. 525 



all the eyes of Limulns are monostichous ; the central eyes of the former 

 group and other Arachnids, so far as known, diplostichous. With the 

 exception of the lateral eyes of Limtdus, which are polymeniscous, or more 

 probably composed of a number of distinct ocelli, Arachnidan eyes are 

 always monomeniscous. Vitrellae are absent. The eyes of Linn this 

 and Scorpions are retinulate. Each retinula in the central eyes of the 

 latter consists of five cells surrounding a five-fluted rhabdome. The 

 grouping of the cells is not so distinct in the lateral eyes. In Limulus 

 the central eyes have groups of five retinular cells, but the grouping is 

 sometimes quite irregular ; the lateral eyes have retinulae with ten cells, 

 each retinula corresponding to a single lens (supra]. Curious differences 

 of structure have been observed in certain Spiders between the anterior 

 and posterior eyes of one and the same set 1 . In Araneidae auditory 

 hairs occur on the palpi, tibiae and tarsal joints, and a sensory organ, 

 probably olfactory or gustatory, on the anterior surface of the basal joint 

 ( = maxilla) of the palpi or second pair of limbs. 



The digestive tract takes a straight course. The stomodaeum is 

 muscular in Linguatulina. The part that traverses the oesophageal nerve 

 collar is generally contracted, the anterior oral end as well as the posterior 

 end being frequently dilated, and in the Araneidae (and others) the latter 

 forms the ' sucking stomach.' Salivary glands, paired or unpaired, are 

 generally present. They are absent in Limulus. The mesenteron is of 

 considerable length. Its thoracic portion in Araneidae is furnished with 

 five pairs of caeca which enter the bases of the limbs. It has lateral 

 tubular outgrowths or glands, sometimes forked or ramified, and then 

 forming a continuous (?)mass in Scorpionidae and Limuhis. The numbers 



1 The differences in question are e. g. in Epeira that the visual cells or retinophorae of the 

 anterior eyes are very numerous and slender, and their retinidia terminal ; of the posterior eyes 

 relatively few, stout, and their retinidia distinctly axial, whilst the retinophoral nuclei are conse- 

 quently situated to the outer side of the retinidia, as in the polymeniscous eyes of Insecta and 

 Crustacea. There can be little doubt that the retinidia of the posterior eyes are compound, and 

 probably double as in Mollusca. In Lycosa and Salticus the ommateum of the posterior eyes is 

 feebly concave, and does not extend laterally as far as do the vitreous cells. The outer ends of the 

 retinophorae in Lycosa are swollen, and contain the nuclei ; whilst in Salticus they are lengthened out 

 and bent horizontally, so that the nuclei form a circumferential layer. Graber states that the vitreous 

 cells = corneal hypodermis, are present in the lateral eyes of Scorpions. Patten regards the eyes of 

 Limulus and Scorpions as of a peculiar type altogether, and as divergent in the same direction, 

 Limulus more than Scorpions. They start with the fusion of a number of ocelli, which is not the 

 case with the compound eyes of other Arthropods. It may be noted that the basement membrane 

 of the hypodermis extends across the eye beneath the corneal hypodermis ( = vitreous cells) in the 

 eyes of Spiders, the central eyes of Scorpions, and of Limulus. In the last-named there are also a 

 number of intrusive connective tissue elements. Lankester and Bourne figure the hypodermic base- 

 ment membrane as passing beneath the lateral eyes of Limulus and Scorpions. In Spiders a mem- 

 brane also extends in a similar way beneath the eye, and is stated to be continuous with the hypodermic 

 basement membrane. The rhabdomes in Lankester and Bourne's account of Limulus and Scorpions 

 would correspond on Patten's view to the vitrellae so called of Insecta, &c. See note,* p. 452, where 

 Patten's terms are explained. 



