492 THE ANIMAL KINGDOM. 



other Arthropoda the muscles are striated, and are disposed in separate 

 fascicles. Ciliated epithelium is universally absent, except, perhaps, in the 

 funnels of the nephridia, &c. of Peripatus. The integument is remarkably 

 poor in glands. Connective tissue is variably developed, to the greatest 

 degree, perhaps, in the higher Crustacea, in Scorpio and Limulus among 

 Arachnida, and as a fatty tissue in Insecta. 



The supra-oesophageal ganglion in its simplest form or archi-cerebrum 

 innervates the eyes and special sense-organs of the head, as in the Arachnid 

 Limulus. In other Arthropoda (Scorpions ?) it is probably always fused 

 with other ganglia, and is a syn-cerebrum. In Insecta it gives origin in 

 addition to the nerves of special sense, to the stomato-gastric sympathetic 

 system, and in most Crustacea to the nerves of both pairs of antennae (see 

 p. 187) and to the stomato-gastric in part. The ganglia supplying the 

 jaws are fused together into an infra-oesophageal ganglion, and the suc- 

 ceeding ganglia, typically one to each somite, frequently undergo greater 

 or less concentration. Each ganglion consists of a right and left half, 

 which are sometimes distinct, and only connected by transverse com- 

 missures (a few Crustacea}. Successive pairs of ganglia are united by two 

 longitudinal commissures, which may retain their primitive distinctness or 

 become united in one sheath. The structure of the nervous system of 

 Peripatus is unique among Arthropoda. 



Eyes are confined, with two exceptions, to the head. The cuticle 

 corresponding to the eye is thickened to form one or many lenses ; hence 

 mono- and poly-meniscous. The hypodermic cells beneath the thickened 

 cuticle constitute the ommateum, and remain either in a single or form a 

 double layer ; hence mono- or diplo-stichous. The monostichous ommateum 

 is said to be apostatic when cup-shaped, and epistatic when in close 

 contact with the cuticle and following its curvature. In the diplostichous 

 eye the anterior layer forms the vitreous layer. Its cells either retain their 

 regular arrangement or become disposed in groups or vitrellae, which 

 develope a transparent vitreous body. The posterior layer forms the 

 retinal layer ; when its cells are collected into groups or retinulae it is said 

 to be retinulate. Each cell in a group generally forms a clear visual rod 

 or rhabdomere. The rods belonging to a group of cells commonly fuse 

 into a rhabdome. The retinal cells are often more or less pigmented. 

 Pigment cells are also present between them, and between the vitrellae. 

 They are derived from the hypodermis, or perhaps in some instances from 

 intrusive mesoblast, hence the eye is auto- or exo-chromic. A mono- 

 meniscous eye may be retinulate. In a polymeniscous eye a single lens- 

 facet, a vitrella, and retinula constitute an * element,' or the two latter, i. e. 

 vitrella and retinula, an ommatidium. The vitreous cells retain the power 

 of forming cuticle, since that structure is shed at each moult over the eye 

 as elsewhere. In the higher Crustacea, in Scutigera among Myriapoda, and 



