Introduction to Animal Morphology. 29 



seen within, as in Crustacea, In Arachnida there is only one 

 corneal lens for a number of crystal cones, and the same 

 occurs in the stemmata or single eyes of Insects. In some 

 Spiders there are muscular (iridal ?) fibres among the pigment, 

 and a rudimentary reflector or tapetum. Sometimes in 

 Crustacea, muscular fibres exist capable of drawing the cornea 

 to the cones, a rudimentary accommodating apparatus. 

 These eyes capable of receiving and recognizing images are 

 called Idoscopic {Jourdain). 



The lowest animals live in water, hence pseudopodia 

 or cilia are sufficient means of locomotion. Some 

 Gregarinae have a special contractile layer like the 

 axis of the stalk of Vorticella. Definite muscular 

 fibres appear in Hydrozoa, first as processes from the 

 neuro-muscular cells of Hydra attached to the ectoderm, 

 and these become arranged in strata. With antimeral 

 growth, muscle bands assume a symmetrical, and with 

 metameral, a homodynamic symmetry. When ap- 

 pendages form from the axis they have special bands 

 of these strata attached to them, and thus a muscular 

 system grows in complexity. Each muscle carries its 

 proper nerve, so the peripheral nervous system grows 

 part passu. According as the muscle is attached to 

 the dermis or to the inner connective tissue, it is 

 called exo- or endo-skeletal. The endoskeleton itself 

 first appears as a fibrous or cartilaginous investment 

 of the digestive and sense organs in Molluscs, but is 

 fully developed in Vertebrata. 



The assimilative organs are differentiated next to 

 the relational. A naked plastide can take in and 

 digest food and eject effete particles at any part of its 

 mass, but the formation of a cuticle renders the pre- 

 sence of a mouth necessary. This appears first in 

 ciliated Infusoria, and may open directly into the 



